Monospecific diatom cultures (Thalassiosira punctigera and Skeletonema costatum) were incubated in rotating cylinders together with clay suspensions, present in a range of concentrations (5-100 mg kaolinite L-1) and as different minerals (50 mg L-1 kaolinite, smectite, illite, and clay-sized quartz powder). The addition of lithogenic suspensions to diatom cultures accelerated the formation of visible aggregates in the roller tanks by a factor of >3. Aggregate size decreased and density increased proportionally to the amount of kaolinite added to the diatom cultures. In the presence of kaolinite and illite, aggregate sizes were smaller and sinking rates lower than in the presence of smectite and quartz. The influence of lithogenic matter on the sinking velocities of aggregates was ambiguous. Compound aggregates sank faster with increasing amounts of lithogenic matter present in cultures of T. punctigera until a certain ratio between lithogenic and biogenic material was reached; further increasing the amount of lithogenic matter did not increase sinking rates significantly. In contrast, increasing the concentration of kaolinite added to cultures of S. costatum could decrease sinking velocities of the evolving compound aggregates. This nonlinear behavior is argued to be primarily a function of aggregate composition on aggregate sizes and excess densities. Although the possibility of a mutual acceleration of vertical flux of algae and clay is confirmed, the results show that the presence of lithogenic material could also decrease the downward flux of phytoplankton biomass.