1. The pH dependence of the Ca2+ sensitivities of isometric tension and stiffness was investigated at 10 and 15 degrees C in skinned single fibres from rat and rabbit fast‐ and slow‐twitch skeletal muscles. Stiffness was determined by recording the tension responses to sinusoidal length changes (3.3 kHz); peak‐to‐peak amplitude of the length change was monitored by laser diffraction and averaged approximately 1.3 nm (half‐sarcomere)‐1. We have assumed that stiffness provides a measure of the number of cross‐bridge attachments to actin. 2. At maximal Ca2+ activation, stiffness was depressed by 22 +/‐ 2% (mean +/‐ S.E.M.) in fast‐twitch fibres but was unchanged in slow‐twitch fibres when pH was lowered from 7.00 to 6.20. As reported previously, maximum tension was depressed by 20‐45% at low pH, with the effect being greater in fast‐twitch compared to slow‐twitch fibres. 3. In fast‐twitch fibres at 10 and 15 degrees C the Ca2+ concentrations for half‐maximal activation of tension and stiffness were increased at low pH. In slow‐twitch fibres, similar effects were observed at 15 degrees C, but at 10 degrees C there were no changes in the Ca2+ sensitivities of tension and stiffness when pH was lowered. 4. Linear relationships between relative tension and relative stiffness were obtained at all temperatures, with slopes of 1.04 +/‐ 0.01 at pH 7.00 and 0.76 +/‐ 0.01 at pH 6.20 in fast‐ and slow‐twitch fibres, indicating that force per cross‐bridge attachment is similarly reduced at low pH in both types of fibres. 5. In both fast‐ and slow‐twitch fibres, rigor tension (no added ATP or creatine phosphate; pCa 9.0) was depressed by 35 +/‐ 7% and stiffness by 12 +/‐ 4% when pH was reduced from 7.00 to 6.20. Since cross‐bridge cycling is inhibited in rigor the effect of low pH to depress rigor tension suggests that pH directly modulates the strength of the bond formed between actin and myosin. 6. The effect of low pH to reduce the apparent number of cross‐bridge attachments during maximal Ca2+ activation in fast‐ but not slow‐twitch fibres could account for the greater H(+)‐induced depression of maximum force in fast‐twitch fibres. In both fibre types, the decrease in the number of cross‐bridge attachments at submaximal concentrations of Ca2+ may in part account for the loss in Ca2+ sensitivity of tension at low pH, due perhaps to a reduction in co‐operative activation of the thin filament by bound cross‐bridges.