The thermal sensitivity of the common brown shrimp Crangon crangon L. and the prawns Palaemon serratue (Pennant) and Palaemon elegana (Bethke) was investigated by studying their behaviour both in their natural environment and in the laboratory. The annual migratory cycle of Crangon, previously reported by others, was confirmed in Swansea Bay, S. Wales. The shrimps migrate offshore in autumn and start to return in early spring. The return migration is interrupted by an offshore breeding migration in April. A correlation was shown between the timing of the autumn offshore migration and the time when shallow water near the shore became colder than deeper offshore water. The return migration coincided with the temperature reversion. Laboratory observations revealed that temperature changes affect the activity of shrimps. It is suggested that this effect provides a mechanism for the annual migratory cycle involving differential tidal transport. Lethal temperature values were determined for different sex and size categories of Crangon in summer, spring and winter, and some comparisons were made with. P. serratus and Carcinus maenas (L.). The relevance of lethal temperatures is discussed in relation to the temperatures experienced by shrimps and prawns in their natural environment. True thermal discrimination was demonstrated is Crangon using choice chambers. Shrimps acclimated at 20-22°C avoided temperatures of 28.8°C and 9.0°C in a Y-maze, and showed a preferendum at 15-22°C in a total range of 15-30°C. A nociceptive sensitivity to temperature was also shown by means of localized and generalized methods of stimulation. Both thermal sensitivity levels and nociceptive thresholds changed with corresponding changes in acclimation temperature. The potential sensory repertoire of the antennal flagellum of Crangon and Palaemon spp. was described by the use of light and scanning electron microscopy. Four kinds of possible receptor were found: setae in groups, single setae, pits with a central dome and simple pits.