Two African populations of Ceratitis capitata (Kenya and Réunion Isl.) and two Mediterranean ones (Sardinia and Procida Isl.) have been studied for genetic variability at 25 loci by electrophoresis. Wright's FST, Slatkin's Nm* gene flow estimator, Nei's distance (D) together with measures of variability such as H, P, A have been used to compare the population from Kenya with the other three. Parameters using gene frequencies (FST, D, Nm*) indicate the presence of substantial geographic heterogeneity, largely attributable to genetic drift and correlated with dispersion of the medfly from its source area (Subsaharan Africa) to the periphery. The Kenyan population has high genetic variability (assessed by H, P and A), as might be expected given its native status. Significant gene flow estimates between Kenya and the derived Mediterranean populations supports the hypothesis of recent colonization. Part of the geographic heterogeneity is related to the presence of fixed alleles in the more differentiated Réunion population although it maintains the genetic attributes of the ancestral population. Selection or other forces may have played an important role in the differentiation of this population.