Larvae of Trachypachus Motschulsky and both species of Systolosoma Solier are described. External and internal features are interpreted phylogenetically. A sistergroup relationship between the genera Systolosoma and Trachypachus is supported by the results of the study. Larval autapomorphies of the Trachypachidae are the presence of a ventral sensorial field on antennomere III which replaces the lateral appendage, the presence of additional apical setae on maxillary palpomere III, and short and horn-shaped urogomphi. Autapomorphies of Systolosoma are the presence of a large sensorial area on antennomere IV, the kidney-shaped, convex sensorial field on antennomere III, the increased number of additional setae on maxillary palpomere III, the high degree of reduction of the urogomphi, the reduced primary setae LA7 (ligular region of prementum) and UR9 (apex of urogomphi), and fused sternal sclerites. A possible autapomorphy of Trachypachus is the presence of two sensorial areas on the dilated apical part of the distal maxillary palpomere. The sistergroup relationship between Trachypachidae and Dytiscoidea is strongly supported by the presence of caudal tentorial arms in larvae of both trachypachid genera. 1st instar larvae of Systolosoma lateritium are fully as large as 2nd instar larvae of S. breve. This, in combination with a similar size of the adults, suggests that the number of larval instars has been reduced in S. lateritium.