Diamondback moth, Plutella xylostella (Linnaeus) (Plutellidae), was last studied in South Africa in the Pretoria North district in 1947 by Ullyett. In the present study the incidence and seasonal history of P. xylostella were studied in the same area. Populations were low (maximum 0.4 larvae/plant) throughout the study period (February 1991 to February 1992) and declined during winter. Only one parasitoid, Apanteles halfordi Ullyett (Hymenoptera: Braconidae), was present throughout the study, in contrast to a complex of 14 insect parasitoid species recorded by Ullyett. The latter complex included a Diadegma species (Hymenoptera: Ichneumonidae) and A. halfordi which Ullyett regarded as the most and second most important species controlling P. xylostella. The present study revealed that P. xylostella did not cause economic losses in this study area, and was apparently controlled by a combination of chemical measures and A. halfordi which caused high mortality (up to 90%) at the low host-population levels. At a second study site at Dinokana in the western Transvaal, however, epidemic outbreaks of P. xylostella occurred from October to December 1991. The average numbers of larvae/plant for middle-aged and mature cabbages were 36 and five respectively, and caused total crop loss. At this study site, regular and excessive chemical applications had been made during the previous two years (1990 and 1991) and recently transplanted two-week-old cabbages averaged 39 eggs, 24 mines and 29 larvae per plant despite regular chemical applications. Plutella xylostella thus appears to have developed resistance to pesticides in this region of South Africa. Features of P. xylostella and A. halfordi that make them an ideal biocontrol association are examined and discussed.