The flash-stimulated luminescent response (FSLR) of euphausiids is a useful tool for the investigation of their bioluminescent behaviour. This paper describes the investigation of the effects of temperature upon a series of such FSLRs produced in individuals of Thysanoessa raschi (M. Sars) by three stimulations with an electronic photofiash at 20 min intervals. The following parameters of the response were measured: responsedelay, responseduration, the sum of these two {delay + duration) and the proportion of stimulations which produced a response. Initial experiments showed that all response delays were significantly different at 10°C and at 5°C, while second andthird response delays were not different from each other. The effect of a range of temperatures from 0° to 20°C on FSLR delay and duration is described. Regression lines based on the Arrhenius equation were fitted to the results. These showed that all response parameters decreased with increasing temperature, although Q10 varied from — 7.8 in the case of thefirst response delay to — 1.3 in the case of second/third response delay. This difference was the most marked effect of the second stimulation; the third stimulation caused no additional change. Below 10°C the second delay was shorter than the first delay. The independent response parameters were found to be delay and delay + duration. Duration resulted from the interaction of these two. These results are discussed and compared with the interpretation of the results of similar experiments by Kay (1965, 1966) onMeganyctiphanes norvegica.