To elucidate the molecular adaptation mechanisms of enzymes to the high hydrostatic pressure of deep sea, we compared the structure, stability, and function of dihydrofolate reductase (DHFR) from a deep-sea bacterium, Moritella profunda (mpDHFR), with those from Escherichia coli (ecDHFR). The backbone structure of mpDHFR almost overlapped with that of ecDHFR. However, the structural stability of both DHFRs was quite different: mpDHFR was more thermally stable than ecDHFR but less stable against urea and pressure unfolding. The smaller volume changes due to unfolding suggest that the native structure of mpDHFR involves a smaller amount of cavity and/or an enhanced hydration compared to ecDHFR. The enzymatic activity of the wild-type ecDHFR decreased under high pressure, but mpDHFR showed the maximum activity around 50 MPa, and the D27E mutant of ecDHFR exhibited pressureactivation. The inverted activation volumes of these DHFRs suggest the changes in the cavity and hydration of the transition-state in the rate-determining step of the enzymatic reaction. Since the cavity and hydration depend on the amino acid side chains, DHFR could adapt to the deep-sea environment without altering their backbone structure. The results indicate the importance of cavity and hydration on the molecular adaptation of proteins to the deep-sea environments.