The deep sea is characterized by low temperatures and high hydrostatic pressures. These factors are known to influence biochemical processes. All of the proteins and membranes of deep- living fishes experience these parameters, and thus these factors may serve as important selective forces shaping the molecular evolution of organisms colonizing and inhabiting the hydrostatic pressures typical of the deep-water environment in shaping the evolution of (1) membrane function, (2) the structural stability of proteins, and (3) the function of enzymes. Comparisons of the function of membranes and proteins of related teleost fish species which occur at different depths in the ocean will be made under conditions of varying hydrostatic pressure. These studies are designed to identify depth-related differences and to provide an understanding of which steps in biochemical processes are susceptible to pressure perturbation, the importance of these perturbations for colonization of deep water, what types of adaptations are employed to cope with these pressures, and what role depth per se plays in speciation and evolution in the marine environment.