The rate of light-saturated photosynthesis in 3 marine algae [ Phaeodactylum tricornutum Bohlin, hrltzschia closterium (Ehrenberg) Smith and Dunaliella tertiolecta ButcherJ varies during growth in batch culture. The photosynthetic rate declines most rapidly during growth at the higher temperatures. Because of these changes in photosynthesis rate, the previously reported enhanced photosynthetic abilities caused by growth at lower temperatures (generally interpreted as evidence for higher enzyme levels) can only be observed when measurements are made late in the exponential phase or after the onset of the stationary phase of growth. When allowance is made for the earlier peak of photosynthetic ability in cultures growing at higher temperatures, there is no evidence for adaptation to lower temperatures being caused by increased levels of the enzymes required for carbon-dioxide fixation. When the changes due to growth in batch culture are taken into account, certain effects of temperature can be recognized. The dry weight: chlorophyll ratio of all 3 algae increases with decreasing growth temperatures. For P. tricornutum and h r. closterium, growth at lower temperatures reduces the cellular content of chlorophyll a, but has little effect on the chlorophyll content of D. tertiolecta. The dry weight:cell-number ratio of /). tertiolecta and P. tricornutum increases with lower growth tem-per&tures, but growth temperature has little effect on the cell mass of hr. closterium. Growth of the 3 algae at lower temperatures does not increase their ability to photosynthesize at these lower temperatures. Rather, it reduces their ability to assimilate carbon dioxide at the higher temperatures.