Four species of biraphid pennate diatoms (Craticula cuspidata (Kiitzing) D.G. Mann, Stauroneis phoenicenteron (Nitzsch) Ehrenberg, Nitzschia linearis (Agardh) W. Smith, and Pinnularia viridis (Nitzsch) Ehrenberg) were studied to determine the effect of temperature changes on their motility and adhesion to the substratum. The pattern of cell movement in response to temperature was similar in all species; the average cell speed increased with temperature, with a maximal speed at 30-35°C. Temperatures > 35OC caused a rapid decrease in cell speed, with cells showing little movement at 40°C; the shape of the temperature profile was nearly identical in all species. The loss of motility above the threshold was not due to cell death as cells exposed to 42°C could recover movement when cooled. In contrast to temperature effects on cell movement, its effect on adhesion for all four species showed distinct qualitative differences. The adhesion of P. viridis and S. phoenicenteron to a glass substrate decreased from 20°C to 35"C, but increased slightly at 45OC. The loss of adhesion in S. phoenicenteron was not quite as severe as in P. viridis, with the latter showing little ability to remain adhered at higher temperatures. Nitzschia linearis demonstrated a higher degree of cell adhesion between 5 and 45°C than C. cuspidata, with both species showing little change in adhesion over the temperature range. Flume studies also demonstrated that P. viridis decreased its adhesion as temperature increased, while S. phoenicenteron maintained or slightly increased its adhesion. The lack of correlation between the adhesion and speed responses to temperature changes and the uniformity in the shape of the temperature/speed profile for all four species suggest that alterations in adhesion characteristics are not directly responsible for regulating cell speed. Both cell speed and adhesion were sensitive to the presence of a second species. Pinnularia viridis displayed lower adhesion in dual-species test samples, and exhibited significantly reduced average speed in the presence of S. phoenicenteron, but not when co-incubated with C. cuspidata. In contrast, S. phoenicenteron showed no change in adhesion in the presence of P. viridis. Additional species had no effect on the cell speed of either C. cuspidata or S. phoenicenteron, while C. cuspidata did show reduced adhesion in the presence of P. viridis. Our results suggest that ecologically important behaviours such as movement and adhesion can respond not only to environmental factors such as temperature, but also to the presence or absence of other diatom species.