A dominant mutation, Sergeant (Sr²), which affects expression of a third stripe on the abdomen of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), was recently isolated and the results of an extended genetic analysis are reported here. In addition, field cage data are presented on the mating competitiveness of strains carrying this mutation, including a genetic sexing strain (GSS). The incorporation of a phenotypic marker, such as Sr², in a Mediterranean fruit fly GSS would complement, and could replace, the use of fluorescent dye for monitoring sterile insect technique programs and increase program efficiency by improving sterile and wild fly identification procedures. The mutation is homozygous lethal, and it is located on chromosome 5 at position 78B and 60D on the salivary and trichogen polytene maps, respectively. The development of the Sr² GSS is described here, together with data on fitness and penetrance of the mutation. The field-cage results demonstrated 1) random mating between males and females of laboratory strains carrying or lacking Sr², 2) comparable sexual compatibility of Sr² and non-Sr² GSS with two wild populations, and 3) comparable mating competitiveness of males from Sr² and non-Sr² GSS with wild males. From these results, no significant effects of the mutation on the sexual competitiveness of strains carrying Sr² were detectable. The findings of this study point to a satisfactory overall genetic as well as mating behavior profile of Mediterranean fruit fly strains carrying the Sr² mutation and clearly warrant further evaluation of this mutation in Mediterranean fruit fly GSS.