SummaryLaboratory infections ofPorphyra perforatathallus discs with zoospores ofPythium marinumshowed that the colonization rate is markedly influenced by temperature, salinity, and the reproductive stage of the algal discs. The most rapid colonization rate occurred on discs obtained from reproductive areas of the alga, or areas immediately adjacent to these, when the discs were in diluted seawater with a salinity of 24%. and at 20 C. All species ofPorphyraexamined were capable of being infected in the laboratory as wasPorphyrella gardneri.