The effect of pyrethrum thrips, Thrips nigropilosus Uzel., on yield of pyrethrum (Chrysanthemum cinerariaelolium Vis.) was investigated using three densities of thrips and three pyrethrum clones. Clones differed in their response to increasing thrips-days. Clone 4331 was most sensitive, followed by MA/71/423 and SB/66/107. Yield reduction by T. nigropilosus was caused by reduction in flower numbers; the pyrethrin content of the flowers was not affected. Taylor's Power Law showed that sampling parameters were the same for all clones and insecticide treatments. Economic injury levels for the three clones were determined. Pooled over clones, the economic injury level based on control costs in the study was 416 thrips-days 25 leavel-1. This low level justifies control of thrips as soon as they are discovered on pyrethrum plants. It is suggested that clonal selection for resistance to thrips should be an important aspect of control of thrips in pyrethrum.