We investigated the tolerance of adult specimens of the shallow-water shrimp Palaemonetes varians to sus- tained high hydrostatic pressure (10 MPa) across its thermal tolerance window (from 5 to 27 °C) using both behavioural (survival and activity) and molecular (hsp70 gene expression) approaches. To our knowl- edge, this paper reports the longest elevated hydrostatic pressure exposures ever performed on a shallow- water marine organism. Behavioural analysis showed a 100% survival rate of P. varians after 7 days at 10 MPa and 5 or 10 °C, whilst cannibalism was observed at elevated temperature (27 °C), suggesting no im- pairment of specific dynamic action. A significant interaction of pressure and temperature was observed for both behavioural and molecular responses. Elevated pressure was found to exacerbate the effect of temper- ature on the behaviour of the animals by reducing activity at low temperature and by increasing activity at high temperature. In contrast, only high pressure combined with low temperature increased the expression of hsp70 genes. We suggest that the impressive tolerance of P. varians to sustained elevated pressure may re- flect the physiological capability of an ancestral species to colonise the deep sea. Our results also support the hypothesis that deep-sea colonisation may have occurred during geological periods of time when the oceanic water column was warm and vertically homogenous.