We present the first data on the interacting effect of temperature and light on primary production of the toxic cyanobacterium Cylindrospermopsis raciborskii in situ. C. raciborskii can be a dominant component of the phytoplankton community in tropical and subtropical lakes and reservoirs. We examined the interacting effects of a range of light (0, 2, 7, 17, 30, and 100% of ambient light) and temperature (20, 24, 28, and 32 °C) conditions, in terms of primary production rate and primary production irradiance model parameters, for a C. raciborskii-dominated phytoplankton community in a subtropical reservoir. Based on 13C-uptake experiments, phytoplankton preconditioned to temperatures between 24 and 26 °C had highest maximum primary production rates (2.25 ± 0.45 μg C μg Chl-a−1 h−1) at 28 °C and lowest at 32 °C (0.58 ± 0.13 μg C μg Chl-a−1 h−1). Temperature also had an effect on the response to light conditions. Phytoplankton preconditioned to a shallow euphotic depth (~2.3 m deep) had the lowest half saturation of primary production, Ik, at 28 °C and highest at 32 °C, while the highest temperature treatment also had the highest level of photoinhibition at 100% of ambient light. This suggests that the cyanobacterial community is adapted to a low light environment under optimal temperature conditions for primary productivity. These conditions are consistent with other studies showing that C. raciborskii is highly adapted to low light conditions. This work demonstrates the importance of considering temperature when comparing calibrated primary production parameters.