Morphological variations of Macrobrachium tenellum prawns from two populations from locations in the Copalita and Colotepec riverine basins in the seashore of Oaxaca, Mexico, were studied with geometric morphometry techniques. The purpose was to find and explain significant differences in shape and size between the cephalothorax and the rostrum. Differences between sexes and populations were intended to prove intra and inter-populations. Results suggest low intrapopulation differences but significant interpopulation variation between specimens from different places, while a regression analysis suggests a higher correlation between shape and size only in males. The higher differences in places between and within sexes could be explained due to differences in environmental pressure. A positive allometry bias in males could indicate a sexual selection mechanism affecting mating.