The genus Perkinsus are parasitic protozoans that cause massive inflammatory responses in infected marine shellfish worldwide. This ultimately leads to great economic losses. This study examined the effects of water temperature and salinity on the formation of prezoosporangia and zoosporangia in order to understand the ecology of the pathogens. The induction of prezoosporangia from trophozoites occurred readily at higher water temperatures (20 and 30°C) and they had larger diameters than those incubated at lower temperatures (4 and 10°C). The formation of zoospores in prezoosporangia was also strongly influenced by water temperature and salinity; prezoosporangia exposed to water temperatures of 20 and 30°C and salinities of 20 and 30 ppt had high rates of zoosporulation, while no or very low rates of zoosporulation were observed at temperatures below 10°C or salinity below 10 ppt. Our data will be useful for the development of strategies to counter P. olseni proliferation in Korean waters.