The undulated surf clam Paphia undulata supports Thailand's largest shellfishery in the Gulf of Thailand, with landings in 1999 recorded at 70000 t (metric tonnes) yr(-1). We report, for the first time, the prevalence of Perkinsus sp. in clams in the Gulf. A monthly survey from January to December 2001 utilizing the fluid thioglycollate medium (FTM) method showed that average monthly prevalence was 84.7% (n = 360). The monthly percentage of infected clams was generally 100%, with low prevalence in May (66.7%) and no infection in September. The monthly mean infection intensity in terms of Perkinsus sp. cells g(-1) tissue varied from 0 in September to 187 759 +/- 18970 (x +/- SE) in October. No obvious annual variation in intensity and prevalence was observed. Prezoosporangia that developed in FTM were 25 to 75 pm in diameter. A few days after incubation in aerated seawater, the prezoosporangia underwent successive binary cell division and formed motile zoospores (2 to 5 microm long). The zoospores were released into the seawater through a discharge tube formed during the 2- and 4-cell stages. Serial semi-thin sections (1 to 4 pm thickness) of clam tissue (n = 120 clams) showed developing trophozoites 3 to 6 pm in diameter within gills, connective tissue, gonads and, especially, the digestive glands. Microscopic features of different life stages indicated that Perkinsus sp. in Thailand closely resembled P. olseni (= P. atlanticus) reported in Australia, New Zealand, Korea, Japan, Spain and Portugal.