A critical factor in the application of SIT to Ceratitis capitata populations is the occurrence of multiple matings by medfly females. In the field, medflies may copulate more than once (Bonizzoni et al. 2002) and the frequency of remating may vary due to both ecological conditions and population density. These observations, along with a strong paternity skew observed in wild populations, led to the formulation of hypothesis on the mechanisms that regulate the use of sperm from different males. To elucidate these mechanisms mating experiments were performed in the laboratory, using fly strains with different internal molecular markers. The results are being evaluated taking into account (i) the duration of mating, (ii) the interval between the first and the second mating, (iii) the quantity of sperm transferred by the first and second male and (iv) the amount of progeny from each male.