Bacterial growth rates in sediments have been calculated from measurements of the rate of incorporation of tritiated 3H- thymidine into DNA. The dilution of isotope in DNA was used to estimate the sum of the pools of thymidine in the sediment and of other cellular precursors of thymidine in DNA. Growth rates of bacteria in the surface zone of seagrass [predominantly Zostera capricorni] bed sediments varied from 3.7 .times. 108 cell divisions h-1 g-1 dry weight of sediment on a hot autumn day to 3.3 .times. 106 cell divisions h-1 g-1 in winter. By combining growth rate measurements using the isotope dilution procedure with biomass measurements, it is now possible to obtain reasonably reliable estimates of bacterial productivity in sediments.