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Seasonal changes in Perkinsus olseni infection and gametogenesis in the Manila clam, Ruditapes philippinarum, were investigated over a 14-mo period at a commercial clam bed on Seonjaedo Island, off the west coast of Korea. The prevalence, infection intensity, and gametogenic stages were histologically evaluated. The prevalence and monthly mean infection intensity ranged from 38 to 97% and 0.77–2.48 on a 4-point scale. The infection intensity in autumn was significantly higher than in summer (2003 and 2004, P < 0.05). In 2003, the timing of gametogenesis and spawning varied with respect to infection level; uninfected clams matured earlier than moderately or heavily infected clams. In contrast, no such temporal variation in spawning was observed among infection groups in 2004. The resting period of heavily infected clams was longer than that of moderately infected or uninfected clams. A high level of P. olseni infection appears to disrupt the maturation of the clams, although our numerical scoring of infection intensity based on histopathology was not sufficiently sensitive to establish this. [ABSTRACT FROM AUTHOR]