Relationships of the genera included in the family Diastatidae by Duda (1934) are considered and their position in the Drosophiloidea is assessed. A family Campichoetidae is recognised following Griffiths (1972) for Campichoeta Macquart and the inclusion of Euthychaeta Loew and the fossil genus Pareuthychaeta Hennig in that family is confirmed. Contrary to Griffiths (op. cit.), however, the family Diastatidae is retained for Diastata Meigen alone, because of its many differences from the Ephydridae and Risidae to which it is allied. Euthychaeta spectabilis Loew is redescribed and characterised; a lectotype is designated; male and female genital structure is figured. For Campichoeta Macquart the five European species are keyed and new distribution data provided; male genitalia are figured and a lectotype is designated for C. punctum (Meigen). The Palaearctic and Nepalese species of Diastata are revised; 19 species are recognised - 11 of them described as new: 9 European species include nebulosa (Fallén), fuscula (Fallén), costata Meigen, adusta Meigen, ornata Meigen, vagans Loew, cervinala sp.n., boreonigra sp.n. and flavicosta sp.n.; 1 from the Far East, ussurica Duda and 9 from Nepal, basdeni Okada, okadai sp.n., macula sp.n., lugubris sp.n.,flavipes sp.n., mirabilis sp.n., rustica sp.n., hirsuta sp.n. and dolorosa sp.n. D. ornata is raised from synonymy while D. unipunctata Zetterstedt is placed in synonymy under adusta Meigen, modesta Melander under costata Meigen, fulvifrons Haliday and inornata Loew under fuscula (Fallén). Male genitalia are figured for all species, female ovipositors for 13 species. Available type material of Diastata has been examined and lectotypes have been designated where applicable: nebulosa (Fallén), fuscula (Fallén), costata Meigen, fulvifrons Haliday, vagans Loew, inornata Loew, unipunctata Zetterstedt and excluded from Diastata - anus Meigen, díadema Meigen and gracilipes Meigen. All names of Palaearctic species proposed in Diastata are discussed; 12 names do not belong to either family but only 4 of them can be positively identified, i.e. anus Meigen = Curtonotum anus (Meigen) (Curtonotidae); leucopeza Meigen and diadema Meigen both = Aulacigaster leucopeza (Meigen) (Aulacigastridae); gracilipes Meigen = Campsicnemus species, probably picticornis Zetterstedt (Dolichopodidae).