Puparia of Glossina morsitans Westw. were collected in the Zambesi valley and contained developmental stages from final phases of third-instar larvae to pupae near to eclosion. These puparia or the emerging adults were exposed to gamma-radiation from a Co60 source at rates between 54 and 122 rad/min. Effective mortality in the pupal stage increased from 23 per cent, at 2,000 rads to 64 per cent, at 15,000 rads, while those deposited in spring (August-October) were more sensitive to gamma-rays than those collected in winter (May-July).

Under described test conditions, 25 pairs of untreated flies produced a mean of 21 pupae in a 28-day test period, insemination averaged 95 per cent., and 56 and 62 per cent, of the males and females survived, respectively. Continuous irradiation reduced reproduction by over 95 per cent, between 8,000 and 15,000 rads wiih males emerging within the first two weeks after treatment; while males emerging during the third post-treatment week were completely sterilised with dosages above 4,000 rads. Treatments given as fractions one and four days apart gave a slightly higher level of residual fertility than that obtained with similar but continuous total dosages. Irradiation of adult males reduced reproduction to an extent comparable with that observed in males emerging from pupae within two weeks of treatment (> 90 per cent, from 8,000 rads). Females treated either as adults or as pupae were completely sterilised with doses of 1,000 to 8,000 rads. Males exposed to 8,000 and 12,000 rads retained their sterility throughout 39- and 45-day test periods, and were fully competitive with untreated males.

Survival of virgin flies emerging from treated pupae was reduced both by increasing dosage from 4,000 to 15,000 rads, and decreasing pupal age at the time of treatment. Mortality during the 28-day mating period decreased with 1,000 and 2,000 rads, increased up to 8,000 rads, and apparently decreased with 12,000 and 15,000 rads. When the doses were fractionated, survival was longer than when treatment was continuous, and mortality was reduced to the level expected from the size of the first fraction. Virgin adult males given doses between 4,000 and 18,000 rads, and females given 2,000 and 4,000 rads, lived longer than the corresponding controls.

The ability of the males to inseminate females was not affected by any of the treatments, and sperm from irradiated males were mobile and apparently behaved normally. Dominant lethality in the males was expressed by the failure to produce pupae in matings with untreated females, and in the treated female fly by the complete failure of oogenesis.