The genus Diacyclops shows a worldwide distribution, with a few strictly cosmopolitan species. Most members are widespread in Eurasia and North America, a few species and subspecies are known from the tropics and the temperate Southern Hemisphere.

The genus seems to be particularly diversified in the Palaearctic but the high concentration could be probably due to the historically intensive research in the surface and subterranean habitats of that area rather than of any zoogeographical meaning: likewise, the lack of exhaustive information for the Oriental and Australian regions could be realated to inadequate and deficient collecting efforts in those countries

From an ecological point of view, the genus Diacyclops is the most successful and diverse of cyclopid genera. It includes land species inhabiting a wide variety of shallow fresh waters, distrophic ponds, temporary water bodies, riverine wetlands, mosses and brackish inland waters; other species occur in large and small natural and artificial lakes and reservoirs, occasionally in rivers; it is also widely distributed in cave and interstitial waters and in groundwater related habitats, whre numerous stygobitic species and subspecies, for the most part belonging to the "Diacyclops languidus-languidoides" group, can be found.

Historically, there has been taxonomical confusion and instability regarding the systematic status of the genus Diacyclops. Some authors (Rylov, 1948; Damian-Georgescu, 1963), due to a morphological overlap between the genera Acanthocyclops and Diacyclops, included both genera into the genus Acanthocyclops.
Considering the structure of leg 5 as the main unifying generic feature of the family Cyclopidae, Kiefer (1927, 1967) subdivided the genus Acanthocyclops into two genera: Acanthocyclops, with the subgenera Acanthocyclops s.str. and Megacyclops, and Diacyclops.
Validity of the separation of these genera was questioned by Mazepova (1978) who recognized only Acanthocyclops in her monograph devoted to the Baikal cyclopids.

Following Kiefer's suggestion, Monchenko (1985), Dussart & Defaye (1985) and, more recently, Einsle (1993) definitely accepted both Acanthocyclops and Diacyclops as valid taxa.
Nevertheless, recent progress in cyclopoid research in North America and the discovery of new species with unusual structure and armature of leg 5 (Reid, 1994; Boxshall et al., 1993) have further confused the status and definition of the genus, proving as well that the systematic relationship and biogeographical patterns within the family Cyclopidae should be further elucidated.

There are presently 118 named species and subspecies of Diacyclops, most of them stygobitic or stygophilic inhabitants of different groundwater systems.

Diacyclops alabamensis Reid 1992 [VIII; K]
Diacyclops albus Reid 1991 [VIII; L1]
Diacyclops antrincola Kiefer 1967 [I 5bc, I 7f, I 8cd, I 11; B, K]
Diacyclops balearicus Lescher-Moutoué 1978 [I 2; I]
Diacyclops belgicus (Kiefer 1936) (=D. languidus belgicus) [II 3-8; a, A]
Diacyclops bernardi (Petkovski 1986) [VII, VIII; A]
Diacyclops biceri Boxshall, Evstigneeva & Clark 1995 [III; L1]
Diacyclops bicuspidatus lucanus Pesce & Galassi 1985 [I 5c; K]
Diacyclops chakan Fiers & Reid 1996 [VII 2; a]
Diacyclops chappuisi Naidenow & Pandourski 1992 (+) [II 8; A]
Diacyclops charon Kiefer 1931 [I 7a; B]
Diacyclops chrisae Reid 1992 [VIII; L1]
Diacyclops clandestinus (Kiefer 1926) [I 23, I 5a, I 7b, I 9ab, I 10, I 11-12, II6-8, III 1; K, T, L1, a, B]
Diacyclops cohabitatus Monchenko, 1980 [Ukraine, Austria; wells, caves]
Diacyclops crassicaudis cosana Stella & Salvadori 1954 [I 5c, I 6b; B]
Diacyclops crassicaudis cretensis (Kiefer 1928) [I 8cdf; B, K]
Diacyclops crassicaudis fontinalis (Naidenov 1969) (= D. fontinalis?) [I 9b; a]
Diacyclops crassicaudis lagrecai Pesce & Galassi 1987 [I 5c; K]
Diacyclops crassicaudis trinacriae Pesce & Galassi 1987 [I 5c; K]
Diacyclops cristinae Pesce & Galassi 1987 [I 5c; K]
Diacyclops danielopoli Pospisil & Stoch, 1999 [Austria; phreatic]
Diacyclops dimorphus Reid 1994 (+) [VIII; L1]
Diacyclops dispinosus Ishida, 1994 [Japan; springs]
Diacyclops felix Pospisil & Stoch, 1999 [Austria; phreatic]
Diacyclops haemusi Naidenow & Pandourski 1992 [II 8; a]
Diacyclops harryi Reid 1992 [VIII; a]
Diacyclops humphreysi Pesce & De Laurentiis 1996 [VI 3; a]
Diacyclops hypnicola (Gurney 1927) [I 5a, I 8c-f, I 9b, I 10, II 1; a, K, L1]
Diacyclops ichnusae Pesce & Galassi 1985 [I 6b; K]
Diacyclops ichnusoides Petkovski & Karanovic, 1997 [Balkan Peninsula, lake Ohrid; interstitial]
Diacyclops imparilis Monchenko 1985 [III; a]
Diacyclops insularis Monchenko 1982 [III; L1]
Diacyclops iranicus Pesce & Maggi 1982 [III 3; K]
Diacyclops jeanneli jeanneli (Chappuis 1929) [VIII 5; B]
Diacyclops jeanneli putei Yeatman 1943 [VIII 1; a]
Diacyclops karamani (Kiefer 1932) [I 7d; A]
Diacyclops kyotensis Ito 1964 [III 5; a]
Diacyclops languidoides languidoides (Lilljeborg 1901) s.l. [I, II, III, VIII; a, K, L1, A, T]
Diacyclops languidoides anatolicus Pesce 1992 [III; K]
Diacyclops languidoides aprutinus Pesce & Fabrizi 1979 [I 5c; K]
Diacyclops languidoides badeniae Kiefer 1933 [I 3, III 1; L1, K, b]
Diacyclops languidoides cavernicola (Lindberg 1950) [I 10; B]
Diacyclops languidoides goticus (Kiefer, 1931) [Austria; ground waters]
Diacyclops languidoides japonicus Ito 1952 [III 5; K]
Diacyclops languidoides moravicus Sterba 1956 [II 8; A]
Diacyclops languidoides putealis (Chappuis 1928) [I 4; a, L1]
Diacyclops languidoides suoensis Ito 1954 [III 5; a] (syn. D. clandestinus ?)
Diacyclops languidus (G.O.Sars, 1863) [Austria; hyporheic] (in:Gaviria, 1998)
Diacyclops languidus maisi (Plesa & Buzila, 1998) [Austria; cave waters]
[=Megacyclops (Diacyclops) languidus maisi in Plesa & Buzila, 1998]
Diacyclops lindae Pesce 1986 [I 5c; K]
Diacyclops maggii Pesce & Galassi 1987 [I 5bc; K]
Diacyclops nagatoensis Ito 1964 [III 5: K]
Diacyclops nuragicus Pesce & Galassi 1985 [I 6b; K]
Diacyclops paolae Pesce & Galassi 1987 [I 5c; K]
Diacyclops paralanguidoides Pesce & Galassi 1987 [I 5a; K]
Diacyclops pelagonicus Petkovski 1971 [I 8c, I 9b; K]
Diacyclops pelagonicus saetosus Pandourski 1993 [II 8; A]
Diacyclops puuc Fiers 1996 [VII 2; a]
Diacyclops ruffoi Kiefer 1981 [I 5a; L1]
Diacyclops paolae Pesce & Galassi 1987 [I 5c; K]
Diacyclops paralanguidoides Pesce & Galassi 1987 [I 5a; K]
Diacyclops paolae Pesce & Galassi 1987 [I 5c; K]
Diacyclops paralanguidoides Pesce & Galassi 1987 [I 5a; K]
Diacyclops sardous Pesce & Galassi 1987 [I 6b; K]
Diacyclops skopljensis (Kiefer 1932) [ I 9b; K]
Diacyclops sororum Reid 1992 [VIII; a]
Diacyclops strimonis Pandourski 1994 [II 8; a]
Diacyclops stygius stygius (Chappuis 1924 [I 7e, II 6; B, K]
Diacyclops stygius deminutus (Chappuis 1925) [II 6; B, K]
Diacyclops stygius macedonicus Petkovski 1954 [I 9b; B]
Diacyclops tantalus tantalus (Kiefer 1937) [I 7d; B]
Diacyclops tantalus slovenica Petkovski 1954 [I 7a; T]
Diacyclops trajani Reid 1994 (= Speocyclops minutissimus sensu Petkovski 1954) [I 7; a]
Diacyclops zschokkei (Graeter 1910) [I 4, I 8c-d, I 9b, I 11, II 4,8, III 1; B, a]