The behavior of seawater molecular viscosity is described for actual oceanic conditions. The shear Viscosity (eta(G) = (1.0-1.5) x 10(-7) dbars) is Newtonian and increases with increasing depth (z down to: 2000 in) and decreasing temperature (T from 25 to 5 degrees C). The compression viscosity (eta(K) = (2-4) x 10(-7) dbars) increases with decreasing compression rate (chi) and increasing temperature and salinity, the compression modulus of elasticity (K= (2.2-2.5) x 10(-5) dbar) increasing with volume deformation (chi%) and temperature. The shear, compression, and extensional viscosities (eta(E)) obey Arrhenius equation with flow activation energies Delta E ca. 4.0kcal mol(-1) in shear, and ca. -1.0 kcal mol(-1) in compression. All three viscosities are interrelated, with parameters changing for actual ocean conditions, their values decreasing with decreasing depth, the seawater clusters going from a compressed spheroidal state to an extended ellipsoidal state. The molecular vertical diffusive transport of momentum is controlled not only by the kinematic molecular viscosity (v = eta(G)/rho = (1.0-1.5) x 10(-6) m(2) s(-1), rho being in situ density) but also by its vertical derivative, which is interpreted as a vertical diffusive velocity and provides an advective kinematic viscosity (v* = 10(-14) to 10(-11) m(2) s(-1)). An adequate characterization of the rheological characteristics of seawater in actual oceanic locations is important to improve our understanding of both energy dissipation and the physical environment that affects microorganisms. In particular. since the ocean is characterized by widespread upward motions, the ellipsoidal redistribution of water clusters may have important consequences in the transfer of water-mass, momentum and energy within the upper layers. (c) 2005 Elsevier B.V. All rights reserved.