The ability of microalgae to fix carbon dioxide and convert it into biofuels, foods and other valuable products has drawn a lot of scientific attention in the last decades. In the last years a number of works aimed at understanding the influence of daily and seasonal temperature fluctuations that affect cell metabolism, and thus biomass production efficiency, have been carried out. However the impact of temperature on cell mortality has never been considered, while temperatures higher than the optimal growth temperature are often reached in summer for outdoor cultivation. This paper explores the effect of high temperatures both on mortality and growth for cultures of Chlorella vulgaris in a photobioreactor. Viability was measured with fluorescein diacetate (FDA), and thus mortality and growth rates were estimated, together with chlorophyll a and intracellular contents in carbon and nitrogen. While the fraction of viable cells decreased at higher temperatures, viable growth and mortality increased from 20 degrees C to 28 degrees C. Chl(a):Cresults suggest that temperature induced photoacclimation in the viable fraction of cells at higher temperatures. A Hinshelwood model was fitted to the data and appropriately described the mortality increase with temperature. Mechanisms affecting growth and mortality rates at high temperature are then discussed.