The genetically modified strain of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) VIENNA 8 1260, was developed from the genetic sexing strain VIENNA 8. It has two molecular markers that exhibit red fluorescence in the body and green fluorescence in testis and sperm. These traits offer a precise tool to discriminate between mass-reared and wild males, increasing the effectiveness of sterile insect technique. The reproductive performance of the VIENNA 8 1260 and VIENNA 8 D53- (without the D53 inversion introduced to prevent recombination) was compared at different irradiation doses. The general effect of irradiation on VIENNA 8 1260 followed the same patterns documented in previous publications for VIENNA 8 D53-. Irradiation doses of 80 Gray or greater reduced fertility and induced high levels of sterility in wild females. Fecundity reduction was higher in VIENNA 8 1260 than in VIENNA 8 D53- females. Vertical transmission of the fluorescence gene was confirmed up to the F4 generation. Substerilization in the VIENNA 8 1260 could jeopardize the usefulness of the transgenic strain due to the possible vertical transfer of the fluorescence transgene from the sterile males to the wild flies. A biologically safe higher irradiation dose could result in reduced competitiveness of the VIENNA 8 1260 strain. Mating and remating experiments suggest that Mediterranean fruit fly females exhibit a relative precedence in the use of the sperm: though both sperms are mixed, sperm from the remating is spent first. Results suggest a lower fitness of VIENNA 8 1260 sperm, when compared with sperm from a nonfluorescent bisexual strain, which is consistent with the lower reproductive performance documented for the VIENNA 8 1260 strain.