Artificial infestation of rice plants in cages, damage simulation and protected field trials were conducted to study the reaction to and assess grain yield loss from stem borers in Kenya. Maliarpha separatella Ragonot (Pyralidae, Lepidoptera) was most abundant, Diopsis thoracica Westw. and D. apialis Dalm (Diopsidae, Diptera) were of minor importance during the field trials. Results of damage simulation and artificial infestation indicate that the pre-flowering stage was most susceptible to borers. The effect of M. separatella infestation on rice yield and the relationship between infestation levels and yield loss were established. Borer infestation and rice yield were studied under field conditions at different levels of crop protection. Control measures and economic injury level are discussed.