The occurrence of dark brown melanized spots indicative of the presence of crayfish plague fungus (Aphanomyces astaci Schikora) was investigated in three Finnish lakes inhabited by the introduced American signal crayfish (Pacifastacus leniusculus Dana) in 1979, 1983, 1985-1988, 1990, 1993, and 1994. The crayfish were caught with crayfish traps during August-September, 7-8 weeks after the moult. In one lake in which P. leniusculus population had developed from juveniles imported from Sweden in 1971, the crayfish showed no signs of infection. The lake also contained a population of the native noble crayfish (Astacus astacus L.). The coexistence of the two species for nearly 30 years in the same habitats in this small lake indicates the absence of crayfish plague. In two lakes P. leniusculus populations originated from adults imported from Lake Tahoe, U.S.A., in 1969. In 1979 the prevalence of infected P. leniusculus in the lakes (n = 1,841) was high, 52% in Lake Karisjarvi and 47% in Lake Iso-Majajarvi. By 1988 the prevalence had decreased to only 11% and 12%, respectively. During the 1990s, the number of infected specimens began to increase, reaching 24% in Lake Karisjarvi in 1993, and 18% in Lake Iso-Majajarvi in 1994. The black spots most commonly occurred on the walking legs (40% and 39%, respectively) and the chelae (28% in both lakes). The mean number of spots per crayfish was 2.0 and 2.4; many (43% and 36%, respectively) were > 3 mm in diameter. Crayfish plague does not appear to have affected the P. leniusculus populations in the study lakes in any way, nor have any mortalities been reported in either lake.