Groups of Rhipicephalus appendiculatus ticks were given single doses of irradiation (range 0·25–16 krad) at different stages in their life-cycle: unfed nymphs, engorged nymphs, moulting nymphs and unfed adults. The effects of irradiation were assessed by evaluating the subsequent performances of the ticks when fed on rabbits. Observations were carried through until the viability of egg batches from surviving females had been recorded. Irradiation of unfed and engorged nymphs produced similar effects on the ticks, whilst irradiation of moulting nymphs and adults produced effects which were different from those produced on the other two groups but similar to each other. Ticks were in general unaffected at irradiation doses below 2 krad, but at this level and above, survival rates, mean engorged weights, percentage egg weights and viability of eggs were reduced, and feeding times were extended. Male ticks appeared to be more susceptible to the effects of irradiation than female ticks. Males irradiated at 4 krad were sterile, but since they survived doses considerably higher than this they might well prove suitably competitive in further trials should a sterile male release be considered feasable. The possibility is also considered of irradiating Theileria parva parasites in ticks and thus altering the parasites in some way that could be used in studies of immunisation of cattle against East Coast fever.