Long-term trends in the bacteriological quality of Finnish rivers and lakes were investigated on the basis of some 64,000 observations of thermotolerant coliform bacteria (ThC) and 147,000 observations of fecal streptococci (FS) carried out in 1963-1993. Annual medians of ThC and FS were calculated for rivers and lakes of ten large drainage basins that together cover the whole country. In the lakes of all drainage basins the medians were low, typically less than 10 CFU (colony forming units) in 100 mL, often nondetectable, and trends were rare. Rivers were consistently more polluted than lakes. River water quality was best in sparsely populated areas of the north, where the medians were generally about 10 CFU in 100 mL. In coastal areas in the south and southwest rivers were more polluted and the medians typically exceeded the limit for good swimming water (100 CFU in 100 mL). Although the bacterial medians of rivers fluctuated, they often showed a decreasing trend in the 1960s and 1970s. This decrease occurred simultaneously with water pollution control measures carried out in the country. After the 1980s the decreasing trends, however, did not continue. Both ThC and FS had similar dynamics.