Low concentration of the insecticide Kepone, approaching those found in contaminated James River sediment, were shown to be inhibitory to the growth and oxygen uptake of microorganisms randomly selected from estuarine environments. No significant correlations were noted between growth inhibition by Kepone and cell morphology, aliphatic hydrocarbon utilization, pesticide tolerance, selected enzyme activities, nitrate reduction, and urea hydrolysis. Oxygen uptake by pure cultures grown on glucose or hydrocarbons at cell densities equivalent to 1,000-10,000 cells/ml was decreased by 60-100% at Kepone concentration of 0.02-2.0 mg/liter. Total viable counts from estuarine water or sediments grown aerobically on agar media containing 0.02 mg/liter Kepone were reduced by 8-78%. The inhibitory effect was eliminated partially when sediment populations were grown anaerobically.