Cynolebias rectiventer can be diagnosed from all species of C. perforatus group by the following characters: contact organs restricted to the seven uppermost rays of the pectoral fin in males (vs. contact organs on all rays); ventral process on the autopalatine present (vs. absent); scales absent on the base of the dorsal fin (vs. present); gill rakers slightly curved (vs. bowed); denticles situated on the gill-raker margin (vs. on the entire region); less than one third of caudal fin anteriorly covered by scales (vs. more than one third). It also differs from all congeners by having approximately straight lateral profile of the abdominal region in males (vs. convex) (Ref. 97357).