The effect of substrate (glucose) shock on starved mixed cultures of aquatic bacteria under conditions resembling the natural aquatic situation has been studied. The prevalent organism in the system was a Pseudomonas sp., and short-term loss of viability and long-term loss of several biochemical properties of this strain were observed. Glucose uptake rate and cell size of the population increased immediately after the shock and synchronous growth occurred subsequently. The population which became established after the shock exhibited lower glucose uptake rates at low substrate concentrations than the starved population. The relevance to similar phenomena which occur when aquatic bacteria are transferred to cultivation media is discussed.