The distribution of synaptic sites on multiply innervated muscle fibres was analysed in four teleost fish species (zebrafish, trout, goldfish and stickleback), using acetylcholinesterase histochemistry. Fishes were chosen for this study rather than other vertebrates because of their long period of growth and continuous increase of muscle fibre size. We found that length and diameter of the fibres increase linearly with fish length but that the distance between synaptic sites increases only as the square root of the fish length and of muscle fibre size. This is explained functionally in connection with the increase of the space constant of a muscle fibre that is expected to accompany the increase of its diameter. We suggest that the change in the synaptic distribution is caused by factors associated with the increasingly wider spread of postsynaptic potentials along the growing fibres, as the intersynaptic distance was found to correlate more strongly with fibre size than with other factors, such as age, speed of growth and genetical background.