A turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields. Owing to the stability stratified salinity field in the Pacific, relatively warm, salty water spreads at an equilibrium level, where the plume density equals the ambient density. In contrast, the unstable salinity profile in the Atlantic gives the plume added buoyancy, and it continues to rise to an equilibrium level where it is relatively cool and fresh. These attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer. The equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient. The net entrainment of surrounding seawater is greater in the Atlantic plume model than in the Pacific plume model for the same buoyancy flux. Recent data from the TAG hydrothermal field at 26¿N on the Mid-Atlantic Ridge and data from a vent field on the Endeavor segment of the Juan de Fuca Ridge are compared to model results. The model predicts the penetration height and &thgr;-S characteristics of the Atlantic plume, and it explains some of the differences between the observed Atlantic and Pacific hydrothermal plumes. ¿ American Geophysical Union 1989