Starch gel electrophoresis was used to resolve gene frequencies among populations of screwworm,Cochliomyia hominivorax (Coquerel). The loci examined coded for α-glycerophosphate dehydrogenase, octanol dehydrogenase, and phosphoglucomutase. Flies were sampled by traps widely distributed in Colima Province, southwestern Mexico. Gene frequencies at Odh differed significantly among the 11 populations. Alleles segregating at Pgm and α-Gpdh were homogeneous among populations. There were significant departures from random mating within populations, but no genetic differentiation among populations was detected. The data suggest unrestricted gene flow among populations. Departures from random mating within populations were explained by the pooling of samples from separate breeding units.