The growth (cell division) of Chlorella cells cultured under photosynthetically active radiation (PAR) without near-UV radiation was inferior to that under PAR with near-UV radiation. To elucidate this phenomenon, the relationship between near-UV radiation and ethylene production in Chlorella cells was examined. The suppression of ethylene production by UV radiation suggests that this phenomenon is associated with the production of ethylene. Chlorella (a eukaryotic protista) was found to produce ethylene from methionine via S-adenosyl-methionine (SAM) and 1-aminocyclo-propane-1-carboxylate (ACC) as in higher plants, and the activity of ACC oxidase, a limiting factor of ethylene production, appears to be associated with the growth of Chlorella. A possible mechanism for the action of near-UV radiation on ethylene biosynthesis and growth suppression by ethylene is discussed.