The biology of Phthorimaea operculella (Zell.), a pest of potatoes, is described from observations in Rhodesia in 1967-69, and fluctuations in the total percentage parasitism by indigenous parasites, including Diadegma stellenboschensis (Cam.) (Nythobia stellenboschenis), Pristomerus sp., Chelonus curvimaculatus Cam. and an unidentified mermithid nematode, were recorded. Alternative solanaceous food-plants included Nicandra physaloides, tomato, 2 species of Datura, 2 of Physalis and sweet pepper [Capsicum]. Since its introduction from South Africa in 1967, over 1.5 million examples of the encyrtid parasite Copidosoma koehleri Blanch. (uruguayensis Tachikawa) have been released in Rhodesia for the biological control of Phthorimaea operculella. Over 25 000 examples of the braconid Apanteles subandinus Blanch., also from South Africa, were released. The build-up of parasitism by C. koehleri and the establishment of A. subandinus are described, and it is reported that after May 1969 P. operculella became so scarce that substantial samples became difficult to obtain, so that regular sampling was discontinued. The speed with which C. koehleri spread is believed to have resulted from liberating massive quantities of the parasites on the farms of some of the larger growers of seed potatoes. none