Myosin heavy chain composition of a large number (288) of single fibres from slow (soleus), and fast (superficial part of tibialis anterior, and plantaris) muscles of adult (3-5-month-old) Wistar rats was determined. A combination of SDS--PAGE and monoclonal antibodies against myosin heavy chains allowed to identify four myosin heavy chain isoforms (I, 2A, 2X, and 2B) and to detect myosin heavy chain coexistence. Four groups of fibres containing only one myosin heavy chain (1 myosin heavy chain, 2A myosin heavy chain, 2X myosin heavy chain, and 2B myosin heavy chain), and five groups containing more than one myosin heavy chain (1 and 2A myosin heavy chains, 2A and 2X myosin heavy chains, 2X and minor amounts of 2B (2X-2B fibres), 2B and minor amounts of 2X (2B-2X fibres), and 2A, 2X, and 2B myosin heavy chain were identified and their relative percentages were assessed. Coexistence of fast myosin heavy chain isoforms was found to be very frequent (50% of the fibres in plantaris, and 30% in tibialis anterior), whereas coexistence of slow and fast (2A) myosin heavy chain was very rare. Maximum shortening velocity (V 0) was determined using the slack-test procedure in a subset of 109 fast fibres from the above population. The values of V 0 formed a continuum extending from 2A to 2X to 2X-2B to 2B-2X to 2B fibres. 2A fibres had the lowest value of V 0 and 2B fibres the highest. Only the differences between 2A and 2B and 2A and 2B--2X fibres were statistically significant. Importantly, the variability of V 0 in fibres containing only one myosin heavy chain and in fibres containing a variable proportion of two myosin heavy chain isoforms was similar and, in some case (e.g. 2B fibres), such to encompass the whole range of variation of fast fibres' shortening velocities. The results of this study demonstrate that the large variability in maximum shortening velocity of fast fibres is not due to myosin heavy chain coexistence, and therefore suggest that it cannot be explained on the basis of myosin heavy chain composition.