The flocculation of fine-grained particles depends on collisions due to Brownian motion, fluid shear, and differential settling. Previous experimental work on the flocculation of fine-grained sediments has emphasized the effects of fluid shear. These effects are significant in high-turbulence regions. However, as the turbulence and fluid shear decrease, as, for example, in open waters away from shore, differential settling becomes the dominant mechanism for flocculation. In the present article, previous work on the effects of fluid shear is reviewed. However, the emphasis is on recent experimental work on the effects of differential settling on the flocculation of fine-grained, primarily inorganic particles. The transition in effects between situations where fluid shear is dominant and the other extreme, where differential settling is dominant, was also investigated and is discussed. The sediments used in these studies were natural bottom sediments from the Detroit River inlet to Lake Erie. The tests were initiated with disaggregated sediments and were continued as the particles aggregated and formed flocs. These flocs then grew until a steady state size distribution was reached. In order to reach a steady state the differential settling tests sometimes continued for as long as 30 days; they were done in both freshwater and seawater at sediment concentrations from 1 mg/L to 200 mg/L and with and without treatment to remove organic matter. Floc size distributions as a function of time were determined. From the experiments it is shown that the times to steady state and the steady state median diameters are much larger when differential settling is the dominant mechanism for flocculation than when fluid shear is the dominant mechanism for flocculation. It is also shown that the effects of sediment concentration and salinity are qualitatively similar; i.e., as these quantities increase, both the time to steady state and the steady state floc size decrease. Settling speeds of the flocculated particles were also measured; the settling speeds of flocs are much larger and increase more rapidly with floc diameter when produced by differential settling than when fluid shear is dominant. ¿ American Geophysical Union 1993