We evaluated the effects of temperature and irradianceon growth and photosynthetic activity of Caulerpa prolifera, C. mexicana and C. scalpelliformis, all common species in the eastern Israeli Mediterranean. The growth of these species was negative at 15◦C but optimal at 23–26◦C, averaging 16% at 23◦C and 48% at 26◦C. For all species, the effect of irradiance on growth was seen by a large number of buds (regeneration of new thalli from mother leaves), particularly at the high (120molphotonsm−2 s−1) experimental irradiance. The species most sensitive to high irradiance was C. scalpelliformis, for which growth was negative from 60 mol photonsm−2 s−1 andabove.Photosynthetic rates and photosynthetic parameter s generally correlated with growth,irradiance,and temperature conditions found inthenatural environments for all three species.