The exon-primed intron-crossing (EPIC) PCR technique was used to analyse the size variation at the first intron of the Ceratitis capitata Adh1 gene. A total of 27 samples from 16 natural populations was analysed from five geographical regions in the species range: Africa, Mediterranean Basin, Latin America, Hawaii and Australia. The Adh1 first intron varies extensively in length with at least 18 size variants ranging from 1400 bp to 3450 bp. These variants can be grouped into four distinct size categories: short, medium, long and very long. The majority of these variants are present only in the African populations. Only a subset of the ancestral variants appear to have succeeded in migrating from Africa during the medfly colonization process. The medfly population structure inferred from the intron size polymorphism is congruent with that observed from the analysis of allozyme variation. The geographical dispersal of the medfly from its source area is associated with a gradual and great reduction in intron variability which parallels the trend of decreasing variability evaluated at 26 biochemical loci. The intron phylogenetic tree is in agreement with allozyme data in portraying the dynamic population history of the medfly. Stochastic evolutionary forces such as drift, bottleneck effects and migration seem to have played the major roles in the dispersion pattern of Adh1 intron variation during the colonization of the medfly.