Gene diversity and gene flow in Glossina pallidipes Austen were studied by using allozyme electrophoresis on samples from southern and East Africa. Recorded were 30 alleles segregating at eight loci. Gene diversity was 0.212±0.085 S.E. in four southern African populations and 0.163±0.076 in seven Kenyan populations. All loci were in Hardy-Weinberg equilibrium. Pgm and 6pgd were sex linked. Spatial components of gene diversity were measured by using F statistics. Mating was random within each population. The 'fixation index' F(ST) was 0.133±0.062 among southern African populations. Among the Kenyan populations, F(ST) was 0.159±0.069. F(ST) was 0.238 ±0.051 among all populations. Analysis of variance of gene frequencies showed that 65.8% of the genetic variance lay within populations and 34.2% of the genetic variance lay between Kenyan and southern African populations. These data suggest a strong measure of genetic drift among tsetse populations even in a region where it is thought they are continuously distributed. The causes of this drift require investigation.