Interannual variation of Perkinsus olseni infection intensity in the adult Manila clam Ruditapes philippinarum in Gomso Bay, off the west coast of Korea, was monitored from 1999 to 2000. Infection intensity of P. olseni (i.e., total number of P. olseni cells in unit tissue weight) was determined using Ray's fluid thioglycollate medium assay and Choi's 2-M NaOH digestion. In Gomso Bay, P. olseni monthly infection prevalence ranged from 83 (April 2000)–100%. It was remarkable that of the 18 mo of sampling, the prevalence remained at 100 % in 12 sampling months. Infection intensity of P. olseni in Manila clam ranged 366,001 (July 1999)–2,235,325 cells/g wet tissue (October 1999). The infection intensity recorded in 1999 was significantly higher than the level measured in 2000, suggesting an interannual variation in the intensity (P < 0.05). A very high level of infection intensity observed during the fall (September, October, and November) coincided with a relatively low condition index and mass morality of clams in the bay. Our data suggest that the mass mortality of clams observed during late summer to mid fall in Gomso Bay could be, in part, explained by the high level of Perkinsus infection coupled with the poor physiological condition of clams during the postspawning season.