The rissooidean family Hydrobiidae Troschel, 1857 is supposedly one of the largest gastropod families with more than 400 recent genera assigned. Due to the limited number of robust anatomical characters in hydrobiids, the high degree of intraspecific variation, the unknown phylogenetic significance of anatomical characters, and the high degree of homoplasy in anatomical characters, its systematic is confusing and phylogenetic relationships within the family and with other rissooidean groups are poorly understood. We studied fragments of the COI and 18S genes from representatives of 40 genera to determine if the family Hydrobiidae (as defined by Kabat and Hershler, 1993) is monophyletic, if the family Hydrobiidae could be resolved using these genes, and whether the Cochliopidae are a distinct family as previously suggested. The cluster patterns in the combined 18S + COI tree as well as in the individual 18S and COI trees show that the Hydrobiidae of Kabat and Hershler (1993) are polyphyletic. The ...