The upper thermal tolerance (CT(max)) of beachfleas Orchestia gammarellus (Pallas) collected from a number of different locations in Iceland was determined. Differences were recorded between field populations associated with thermal springs and those from non-thermal sites. A number of reciprocal acclimation experiments (where animals from thermal and non-thermal sites were acclimated to the measured ambient temperatures of thermal (17 and 22 degrees C) and non-thermal (11 degrees C) sites) were performed. Differences between at least one thermal population and a non-thermal population were maintained following this reciprocal acclimation, supporting the hypothesis that population differences were due to non-reversible genetic differences and not local acclimatisation. Animals from one thermal site (Reykjanes) had a mean CT(max)=37.1+/-0.5 degrees C when acclimated at 11 degrees C and 38.6+/-0.3 degrees C when acclimated at 22 degrees C, whereas animals from a non-thermal site (Hvassahraun) had CT(max) values of 35.9+/-0.5 and 37.9+/-0.3 degrees C, respectively. In other cases, differences are best explained by local acclimatisation. Results are discussed in relation to ambient local conditions and the degree of isolation of the different populations.