Microclimate data collection and analyses were made in support of an ecological study of changes in transmission cycles of insect-borne disease during a three-year period of construction of a hydroelectric dam. The dam will cause impoundment of some 300 km2 of tropical forest in the Bayano River Basin, Panama. The pre-impoundment microclimate appeared to be one of strong seasonality with appreciable changes in the wind flow, sensible and latent heat flux, evaporation, rainfall and humidity. Average daily net radiation balance was 106 W m−2 above the forest canopy and 46 W m−2 on the floor of the forest. The moisture balance of the forest indicates an annual rainfall of 2 m of which 1 m reaches the floor of the forest. An appreciable amount of rain is intercepted in the forest canopy. The evaporation and runoff on the forest floor are both about 0.5 m. Average daily wind speeds are light, varying from 8 km day−1 in the rainy season to 126 km day−1 in the dry season. Ecological significance was found in the nocturnal unstable vertical temperature profile of the forest and the associated small updrafts and downdrafts which may permit easier vertical migration of forest insects at night. Rainfall and light wind speeds during 24 h periods of collection seem to appreciably affect the activity of the most abundant man-biting species of insects.