Penicillic acid was isolated from a culture filtrate of Aspergillus sclerotiorum. It had a high in vitro antifungal activity against Phytophthora spp., which has not been previously reported. MICs of penicillic acid were from 1 to 25 mug ml(-1) against Phytophthora spp. Penicillic acid induced abnormal branch formation, apical branching, and swelling in P. capsici, in P. cactorum mycelia contained irregular branching and small spherical swelling at apices, in P. cambivora there was irregular branching and swelling, and in P. drechsleri there was irregular multiple spherical swelling at or near hyphal apices.