The reproductive seasons of 6 species of characoid fishes inhabiting a tropical forest stream in Panama were determined from observations on gonadal condition and size frequency distribution of the populations over an 18—mo period. In this area the relatively mild dry season lasts °4 mo, stream discharge increases moderately during the rainy season, floods are intense but brief and temperature is very stable. Bryconamericus emperador and Piabucina panamensis spawned in temporary tributaries in June with the first floods of the rainy season. Brycon petrosus and Hyphessobrycon panamensis spawned in the dry season. The majority and adult Gephyrocharax atricaudata were mature in most months, but fry appeared in several peaks scattered through both dry and rainy seasons. Roeboides guatemalensis seemed to breed throughout the year but with a dry season peak. Previous studies of tropical freshwater fishes have emphasized the widespread tendency toward a flood spawning pattern but have investigated primarily highly seasonal savanna environments. I suggest that the diversity in the present study is related to the relatively stable habitat. Five hypotheses to explain the diversity in the timing of reproduction are proposed and briefly discussed: reproductive seasonality (1) is controlled by adult or juvenile food availability, (2) is controlled by interspecific competition for food among juveniles, (3) is controlled by competition for spawning sites, (4) is a mechanism for reproductive isolation, or (5) is unrelated to local conditions but is a reflection of earlier evolution of specializations for spawning under particular conditions.