Methanol extracts of corn gromwell (Lithospermum arvense L.) are toxic to larvae of mosquitoes (Aedes aegypti). The active principle of this extract was concentrated by a fractionation sequence that included solvent partitioning, countercurrent distribution and preparative thin-layer chromatography. Fractionation at each stage was monitored by bioassays with mosquito larvae. Activity was concentrated in the hexane-soluble portion of the extract. After further fractionation by 10-stage countercurrent distribution followed by 200-tube countercurrent distribution, one form of activity was concentrated between tubes 80 and 90, as shown by treating larvae at 10 ppm. Other active components were concentrated at tubes 1 and 190.