Stem-growth periodicity of several tree species was investigated in a tropical wet forest of eastern-lowland Costa Rica. The climate of the region is moderately wet and only weakly seasonal. The forest in which the study was conducted is luxuriant, mainly evergreen, and apparently free of recent significant human disturbance. Two sites with differing soil drainage were utilized: a well -drained area and a fresh-water swamp. Radial dendrometers were installed on trees comprising 12 species, and hourly readings were automatically recorded for 20 and 24 mo, respecively. Daily rainfall, air temperature, and relative humidity were monitored; day-length and sun-elevation values were calculated. Five of the eight species studied in the well-drained habitat showed periodic behavior. Their seasonality was mainly annual, but two or four Pentaclethra macroloba had semiannual rhythms. The periodic trees were well synchronized with one another and with the climatic cycle. Al of them exhibitedgrowth cessations that were associated with the drier portion of the year. Each of the two semiannual Pentaclethra underwent an additional interruption during the wet season. Al of the trees monitored in the swamp displayed intermittent stem growth. Oly one or possibly two trees (both Pentaclethra) had annual periods. The remainder showed cycles that were either longer or shorter than a year. The Pentaclethra were fairly well synchronized with one another and with the drier time of year. Cambial activity of the synchronized trees was inversely related to day length, insolation angle, and air temperature at both sites