Five trials were conducted in Kenya in 1962–63 to investigate the effectiveness of a 0·2 per cent, w/w pyrethrum dust, synergised with 5 parts piperonyl butoxide to 1 part pyrethrins, against the stalk borer Busseola fusca (Fuller) by applying it to maize at 20 lb. per acre. In three trials, it was compared with a dust containing 5 per cent. DDT applied at the same rate. Pyrethrum showed an efficiency in controlling B. fusca equal to that of DDT. Both insecticides produced reductions of more than 50 per cent, in the incidence of infestation, and in the final trial there was a significant reduction of over 40 per cent, in the number of cobs damaged by stalk-borer larvae; at the same time, an increase in yield was apparent. Further work is required in order to determine the lowest effective concentration of pyrethrins in the dust, which is likely to be less than 0·2 per cent., and to indicate the best time of application.