1. Inhibition of actomyosin function by decreased pH has been proposed to account for much of the depression of muscle function during fatigue. The clearest support for this hypothesis has been from studies of skinned skeletal muscle fibre mechanics at low temperatures (< or = 15 degrees C). 2. We re-examined the effect of decreased pH (7.0-6.2) on skinned mammalian skeletal fibre mechanics at low (10 degrees C) and high (30 degrees C) temperatures, using recently developed protocols that allow reproducible mechanical data to be obtained at higher temperatures. 3. At 10 degrees C we duplicated previous observations of a significant inhibition of maximum shortening velocity (Vmax) and isometric tension (Po) by acidosis. In contrast, at the higher temperature, we found only a very minimal effect of acidosis on Vmax and a threefold reduction in the decrease in Po. 4. Thus at temperatures only slightly below physiological for mammalian skeletal muscle systems, pH plays a much less important role in the process of muscle fatigue at the cross-bridge level than has been suggested by data obtained at physiologically unrealistic temperatures.