Early attempts to apply the SIT on the olive fly, Bactrocera oleae, were unsuccessful, mainly due to the low competitiveness of the sterile mass-reared males compared with the wild ones. Recently, new efforts are underway in Israel to develop a vigorous and efficient mass-reared olive fly laboratory strain. To understand the genetics of the adaptation process and possibly link the corresponding loss of competitiveness to genetic markers, we followed the fluctuation of genotypic frequencies of ten microsatellite markers during the course of 22 generations of the colonization of a wild Israeli population in laboratory conditions. Effective and observed allele number is halved after 11 generations, so is mean heterozygosity. Practically, there is very little change between F0 and F1, there are substantial changes between F1 and F2–F5, and there is a virtual complete adaptation to the new laboratory environment by F11, because no more changes are observed between F11 and F22. If we assume that the loss of allele number and heterozygosity also reflects the loss of the ‘wild’ character of the colonized strain and, possibly, the loss of a substantial part of its natural vigour, our results indicate that there is an apparent need to refresh a mass-reared colony with wild material at about every five to eight generations. Furthermore, simulation models indicated that while most of the observed allele frequency fluctuations were due to random drift, some alleles were probably under selection.