Understanding inter-population variation and sexual differences in morphology is a fundamental part of taxonomic and biological research. External morphology of swamp barb (Puntius brevis) was compared between sexes and among three populations in the Mekong Delta, including O Mon-Can Tho (n=86), U Minh Thuong-Kien Giang (n=49) and U Minh Ha-Ca Mau (n=77). The results illustrate that color and countable parameters are similar in all populations. Nonetheless, 9 out of 20 morphometric indices are significantly different among three populations (P<0.05), where 6 indices are more important criteria to distinguish among swamp barb populations. Furthermore, the sexual dimorphism of this species is indicated in 16/20 morphometric indices consisting of 5 indices relating egg-carrying characteristics. In particular, the females have higher values of indices relating to abdomen and head parts while most of fin length indices are larger in males than in females. In sum, intraspecific variation in morphometrics of swamp barb is mainly due to sexual dimorphism rather than populations.