SUMMARYAn autosomal translocation in the tsetse flyGlossina austeniwas studied genetically, cytogenetically and for its effects on viability. Flies homo-zygous for the structural change could be identified by outcrossing to wild-type and demonstrating semi-sterility in all the progeny.A cytogenetical analysis of male meioses in samples of pupae which were sibs of the semi-sterile progeny showed them to be structurally heterozygous for the translocation. Matings of the translocation heterozygotes and homozygotes gave the expected progeny ratios, with the exception of a deficit of females classified as translocation homozygotes. This was due to their sterility or inviability. Those female homozygotes which did breed showed a subnormal lifetime pupal production. These deleterious recessive effects were probably due to the translocation itself although the influence of linked loci could not be ruled out. These effects would prevent the mass rearing of this particular translocation for a tesetse control project.Two other stocks which showed semi-sterility were found to carry autosomal translocations and two which currently showed holandric inheritance haveY-autosome translocations. One stock with holandric inheritance of extreme sterility carries a double translocation involving two autosomes and theYchromosome.