The effect of temperatures ranging from 8[degree] to 39[degree]C on the growth of 17 species of marine phytoplankton has been investigated. It was found that growth of all organisms tested took place between 18[degree] and 221[degree]C. There was no growth of the following organisms above 30[degree]C: Mono-chrysis lutheri, Isochrysis galbana, Phaeodactylum tricornutum, Chlamydomonas sp., Chlorella sp. (UHMC), Protococcus sp. (T3), Nitzschia laevis, and Melosira sp. Multiplication at 30[degree]C occurred in Dunaliella euchlora, Chlorella sp. (580), Chlorococcum sp., Platymonas sp., Protococcus sp. (T9), Amphiprora sp., Actinocyclus sp. and Amphora sp. Survival at elevated temperatures is often dependent upon the time of exposure. The absence of growth at an elevated temperature may be due either to an inhibition that is reversible or to an irreversible heat death. At 8[degree]C the rate of growth may be reduced or inhibited, but in no case was this low temperature lethal.