A new genus and species of basal mosasauroid reptile from the Cenomanian (Upper Cretaceous) of Slovenia, previously referred to as the "Trieste aigialosaur," is described. The new taxon is known from only a single specimen including a well-preserved postcranial skeleton and parts of the skull (i.e., left quadrate, left surangular-prearticular complex, and left angular and splenial). Morphological characters of the quadrate and postcranial skeleton distinguish this new taxon from other basal mosasauroids such as Aigialosaurus dalmaticus, Aigialosaurus (=Opetiosau-rus) bucchichi and Carsosaurus marchesettii. Cladistic parsimony analysis of a taxon character matrix composed of 135 characters and 31 terminal taxa resulted in the recovery of 12 equally parsimonious cladograms of 351 steps (CI = 0.48, HI = 0.52, and RI = 0.73). From these cladograms we present a phylogenetic hypothesis on the interrelationships of mosasauroids. Species assigned to Aigialosaurus form the basalmost mosasauroid clade. The new genus described here is reconstructed with Carsosaurus marchesettii in a clade that is the sistergroup to the following clade: (Haasiasaurus ((Halisaurinae, (Tethysaurus nopcsai (Yaguarasaurus columbianus, Russellosaurus coheni)) (Russellosaurina))); the "Dallas aigialosaur," Dallasaurus turneri, is found to be the basal-most mosasaurine. Arguments supporting the importance of pelvic evolution to the aquatic adaptations of mosasaurs are given along with a discussion of recent hypotheses of limb evolution in mosasauroids.