1. The effects of variations in pH between 7.00 and 6.20 on Ca2+ ‐activated tension development and maximum velocity of shortening (Vmax) were examined in skinned single skeletal fibres from rat slow‐twitch soleus and fast‐twitch superficial (s.v.l.) and deep (d.v.l.) regions of the vastus lateralis muscle. 2. At pH 6.50, Vmax was depressed to a similar degree in each of the soleus, d.v.l., and s.v.l. fibres. Lowering pH to 6.20 resulted in a further decline in Vmax in all fibres; however, differences between the slow fibres, identified by SDS‐polyacrylamide gel electrophoresis, and fast fibres were apparent, with soleus retaining a significantly greater proportion of its control Vmax (0.83 +/‐ 0.03 in soleus vs. 0.69 +/‐ 0.03 in s.v.l.; mean +/‐ S.E.M.). 3. Maximum force production decreased significantly as pH was reduced. Peak force at pH 6.50, relative to that at pH 7.00, was significantly greater in soleus (0.80 +/‐ 0.01) than in the s.v.l. (0.75 +/‐ 0.01) fibres. At pH 6.20 these differences between slow and fast fibres were still greater, in that soleus fibres generated significantly greater relative forces (0.73 +/‐ 0.01) than did d.v.l. (0.67 +/‐ 0.02) or s.v.l. (0.63 +/‐ 0.02) fibres. 4. As pH was lowered the tension‐pCa relationship shifted to the right (i.e. to higher [Ca2+]), indicating a reduction in the Ca2+ sensitivity of tension development. The [Ca2+] necessary to achieve half‐maximal tension in both the slow‐ and fast‐twitch fibres increased approximately 5‐fold when pH was lowered from 7.00 to 6.20. Furthermore, in the case of the soleus, the Ca2+ threshold for tension development was 45 times greater at pH 6.20 than at pH 7.00, while in the fast‐twitch fibres, this increase was 4‐fold. 5. Increased [H+] differentially affected the steepness of the tension‐pCa relationship between slow and fast fibres. As pH was lowered, the steepness of the lower portion of the tension‐pCa curve increased in the soleus and decreased in d.v.l. and s.v.l., suggesting that apparent positive co‐operativity of tension development had increased in soleus and decreased in d.v.l. and s.v.l. fibres. 6. These results (1) demonstrate an increased resistance to H+ ion‐mediated contractile dysfunction in slow‐ compared to fast‐twitch single fibres, and (2) support the hypothesis that muscular fatigue resulting from short‐term, intense muscular contraction may in part be related to elevated H+ ion concentration.