The influence of ambient and filtered solar ultraviolet-B (UV)-B radiation and of selenium treatment was determined on photochemical efficiency, respiratory potential measured by electron transport system (ETS) activity, and yield in pumpkins, Cucurbita pepo L. Yield and ETS activity were higher when solar UV-B was filtered out. The results suggested that the decreased yield was related to the UV-B impaired flow of electrons in the respiratory chain. Selenium increased yield under ambient radiation conditions. However, no significant effect of excluding UV-B radiation or of treatment with selenium was observed on the photochemical efficiency of photosystem II (PSII).