The influence of cell age, cell size, cell shape, and mode of colony formation on diatom sinking rates was investigated in 26 experiments with Skeletonema costatum (Grev.) Cleve and Rhizosolenia setigera Brightwell. The mean sinking rate of S. costatum ranged from 0.30 to 1.35 m/day; the absolute minimum and maximum rates observed were 0.15 and 7.39 m/day. Senescent populations were characterized by higher sinking rates. The number of cells/chain decreased with increasing culture age, resulting in the dominance of the population by solitary cells and two‐celled chains after 16 days. The sinking rate of S. costatum was inversely related to the number of cells/chain (colony size) irrespective of culture age. This suggests that the formation of silica rods and chains by S. costatum is an adaptation to flotation.The mean sinking rate of R. setigera ranged from 0.16 to 1.77 m/day; the absolute minimum and maximum rates observed were 0.05 and 22.71 m/day. Post‐auxospore cells sank more rapidly, in general, than pre‐auxospore cells, possibly reflecting the lower surface area : cell volume ratio of the former type. Spineless pre‐auxospore cells sank more rapidly than those having both apical spines. The sinking rate was generally higher in senescent cultures, except where post‐auxospore cells dominated. The post‐auxospore cells frequently exhibited an unique ascending movement off the bottom of the settling chambers that may be related to the light and dark period characteristics of the experiments. This phenomenon became less pronounced, or ceased, with senescence. A similar pre‐auxospore response was observed, with certainty, on only one occasion.From 5 to 14 days would be required by the species to sink 100 m at the absolute maximum sinking rates observed, while from 770 to 2,000 days would be required at the absolute minimum rates. During active growth, an increased buoyancy occurs which appears to be independent of the species’ morphological characteristics, suggesting that the adaptive flotation mechanism or mechanisms at that time may be primarily physiological in nature. However, the influence of morphological features, such as cell and colony size, on the sinking rate progressively increases with senescence. Changes in the ratios of pigment absorbance at 434 : 474 mµ in S. costatum and 352 : 474 mµ in R. setigera suggest that physiological changes also accompany senescence and may contribute to a failure of the adaptive flotation mechanism or mechanisms.