Bentonite (2 g/day) was added to four vessels in the rumen simulation technique (Rusitec) and the effects on the fermentation were compared with four control vessels over a 21-day period. Vessels in both groups were supplied with 20 g/day of a diet containing, per kg, 500 g hay, 299·5 g barley, 100 g molasses, 91 g fishmeal and 9·5 g of mineral/vitamin mix. Numbers of ciliate protozoa were reduced by 69% (40 v. 12·8 ×103/ml) in vessels receiving bentonite. The viable count of bacteria was increased by one-third in these vessels (4·8 v. 3·6 × 108/ml), and ammonia production was 54·7 and 76·7 mg/day in the bentonite and control vessels, respectively. Other effects on fermentation products and metabolic activities were minor, except that the rate of breakdown of [14C]leucine-labelled Selenomonas ruminantium protein was 47% lower in fluid taken from vessels receiving bentonite. Bentonite did not affect the rate of bacterial breakdown by protozoa immediately when it was added to rumen fluid in vitro, although a decline in activity was becoming apparent after 4 h incubation. Microscopic examination of rumen fluid to which bentonite was added indicated that some bentonite was ingested by protozoa, but that its main toxic effect was at the cell surface, causing interference with the motion of cilia and thereby preventing motility of protozoa, particularly the holotrichs.