Fibroblasts of the line 3T3 from swiss albino mice were exposed to ultraviolet A (UVA) irradiation. The cells were synchronized by treatment with nocodazole and mitotic shake-off, and then exposed to UVA irradiation in different stages of the cell cycle. Their photosensitivity varied through the cell cycle, being greatest in the G₂ phase. UVA irradiation was found to induce the formation of multinucleated cells. Cells in the G₁ phase were found to be most prone to multinucleation 15 min after UVA irradiation, while cells exposed to UVA irradiation in S and G₂ phases contained the largest fractions of multinucleated cells 24 h after treatment. The present results indicate that multinucleated cells are formed by fusion of two or more cells shortly after UVA irradiation of early G₁ cells, while impairment of cytokinesis is a possible explanation for the delayed formation of multinucleated cells after irradiation in S and G₂.