Abstract6‐Methylcholesterol and 6‐ketocholestanol do not promote oospore development inPhytophthora cactorum when added as sole sterol supplement of media, and they inhibit development induced by cholesterol. 6‐Fluorocholesterol promotes oospore formation; 6‐chlorocholesterol does not, and neither inhibits cholesterol‐induced development. A strain ofP. cactorum has been found which is exceptional among Phytophthoras or Pythiums examined hitherto in forming numberous oospores with cholestanol. In this strain, 6‐ketocholestanol inhibits cholestanol‐induced oospore formation. These results indicate the importance of C‐6 as a point of interaction with cellular sites controlling oospore formation.