Genetic methods to eliminate females prior to releasing sexually sterilized males in sterile insect technique programmes generally involve the translocation onto the Y chromosome of a readily selectable gene (here designated F). However, such strains often show instability with regard to sexual dimorphism. Concerted attempts to determine the most probable causes of such instability have rarely been successful, and appropriate countermeasures have generally not been implemented. We have developed a computer model which simulates the progression of instability in such strains. Here we simulate the effects of contamination in association with a range of genetic and biological parameters and assess the patterns and rates of progression of the ensuing instability. A single event of contamination alone was found to contribute relatively little to the progression of instability and invariably resulted in an equilibrium within the population. However, strong effects were produced by reduced fitness and mating competitiveness of both the f/f genotype and the translocation carriers, a reduction in the parameters involving the f/f genotype resulting in the virtual elimination of f/f, while a reduction in the translocation parameters had the opposite effect of virtually eliminating the F/ – phenotype. The sex ratio was affected only by a change in the relative fitness of the f/f genotype and the translocation. Several characteristic patterns contributed to distinguish clearly the effects of contamination from those of male recombination in unstable genetic sexing strains. With such knowledge appropriate measures may be taken to counter or alleviate the effects of instability in mass reared genetic sexing strains.