Sediments from 10 lakes for which bacterial counts were available were submitted to particle size analysis (Coulter Counter), both before and after removal of organic material. While these sediments were not so highly aggregated as soils, they contained water-stable aggregates and differed from one another in this characteristic. The counter failed to detect some apparently unstable aggregations visible in the sediment of one lake. The median sizes of the untreated sediment were observed to be inversely proportional to the median sizes of the sediment from which the organic component had been removed. There was good correlation between logarithms of the heterotrophic bacterial population (culture counts) and the median sizes of the particles in untreated samples of the sediment. Estimations of bacterial density, assuming a specific gravity of 1.6 for these sediments indicated that, for the four lakes for which direct counts of bacteria were available, there were from 3000 to 15 000 bacteria per square millimeter of sediment particle surface, or about 1 bacterium for every 70 to 300 μ2.