This communication describes the effect of temperature on photosynthesis and respiration rates of Lissoclinum bistratum (an ascidian)-Prochloron (a cyanobacte-rium) symbiotic system found in the intertidal rock pools of Darwin Harbour, Australia. It was found that the optimum temperature for photosynthesis was 35oC, after which photosynthetic activity steeply declined. Respiration rates, on the other hand, increased steadily from 22o to 40oC. Based on Q10 analysis, the symbiotic system was more than twice as sensitive to temperature changes below ambient (Q10 of about 5) as compared to temperature changes above ambient (Q10 of about 2). The photosynthetic apparatus of the symbiotic system appeared to suffer irreversible damage when it was subject to more than two hours of incubation at 37oC. The results were compared with those of other ascidian-Prochloron systems from the Great Barrier Reef and Palau.