Xenodexia is a genus of poeciliid fish. It contains the single species Xenodexia ctenolepis, the Grijalva studfish, which is endemic to river basins in Guatemala where it lives in both slow- and fast-flowing waters. Females reach up to 4.5 cm (1.8 in) in total length and males are somewhat smaller. It is the only member of the subfamily Xenodexiinae.