Multilocus enzyme electrophoresis data from 26 polymorphic loci (124 alleles) were used to analyze the genetic aspects of the worldwide colonization of Ceratitis capitata (medfly). Eighty-two samples of 17 populations were collected from six regions throughout the species range: Africa, extra-Mediterranean islands (Madeira and Gran Canaria), Mediterranean region, Latin America (Guatemala), Pacific (Hawaii), and Australia. The variability parameters (H, P, A) reveal that the geographical dispersal of medfly from its ancestral source area (East Africa) is associated with a great reduction in variability. The pattern of decreasing variability occurs at two regional levels: in the African-Mediterranean region where the differentiation is gradual, and in the Latin American-Pacific region where some ancestral variability is still present as a consequence of recent colonization. The UPGMA phylogenetic tree, derived from Nei's genetic distances, shows the presence of intraspecific differentiative processes affecting mainly the two island populations, Réunion and Hawaii. The population genetic changes observed in the species range are consistent with both the chronology and the historical circuitous course of the medfly colonization process.