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Using microcosm experiments, we investigated the interactive effects of temperature and light on specific growth rates of three species each of the phytoplanktonic genera Cryptomonas and Dinobryon. Several species of these genera play important roles in the food web of lakes and seem to be sensitive to high water temperature. We measured growth rates at three to four photon flux densities ranging from 10 to 240 mu mol photon center dot m(-2) center dot s(-1) and at 4-5 temperatures ranging from 10 degrees C to 28 degrees C. The temperature x light interaction was generally strong, species specific, and also genus specific. Five of the six species studied tolerated 25 degrees C when light availability was high; however, low light reduced tolerance of high temperatures. Growth rates of all six species were unaffected by temperature in the 10 degrees C-15 degrees C range at light levels <= 50 mu mol photon center dot m(-2) center dot s(-1). At high light, growth rates of Cryptomonas spp. increased with temperature until the temperature optimum was reached and then declined. The Dinobryon species were less sensitive than Cryptomonas spp. to photon flux densities of 40 mu mol photon center dot m(-2) center dot s(-1) and 200 mu mol photon center dot m(-2) center dot s(-1) over the entire temperature range but did not grow under a combination of very low light (10 mu mol photon center dot m(-2) center dot s(-1)) and high temperature (>= 20 degrees C). Among the three Cryptomonas species, cell volume declined with temperature and the maximum temperature tolerated was negatively related to cell size. Since Cryptomonas is important food for microzooplankton, these trends may affect the pelagic carbon flow if lake warming continues.