San Francisco Bay has a high degree of spatial variability in physical properties (e.g. suspended sediment concentrations, water depths, vertical mixing rates) that affect biological pro- cesses. We used this setting to test the hypothesis that light availability is the primary control of phytoplankton growth in this turbid nutrient-rich estuary. In situ incubations (24 h), designed to simulate vertical mixing over the water column at 2 rates, were done at 4 sites. The photic depth to mixed depth ratio (Z,,:Z,) at the 4 sites ranged from 0.12 to 1.1. Phytoplankton growth rates were estimated by 14C assimilation and by changes in cell number. Growth rates were highest (approximately 2 divisions d-I) where the photic depth was large relative to the mixed depth, and small or negalive where Z,:& was small. Growth rate increased with total daily light exposure and fit a hyperbolic function that predicts maximum specific growth rate of about 2 divisions d-' and a compensation irradiance of about 1.4 Einst. m-2 d-l.