Eight phenotypes of phosphoglucomutase (PGM) were detected in abdominal muscle extracts from P. aztecus by starch-gel electophoresis. The observed Phenotypes were assumed to be under the control of 5 allelic genes. This assumption was supported by the observed distribution of phenotypes. There were no significant differences in PGM phenotype distribution between sexes or among samples of shrimp taken from Galveston Bay, Texas, and Vermillion and Barataria Bays, Louisiana, USA.