Conduction velocities of antidromically evoked impulses along the goldfish Mauthner axon were found to be inversely correlated with body length. To test the hypothesis that such a relation is accompanied by a reduction ïn axonal diameter with increasing fish size, Mauthner axon diameters were measured. A parabolic relationship with respect to body length was obtained, axonal diameter being maximal in 9.5 cm fish. To our knowledge, this is the first report of a decrease in conduction velocity and axonal size during growth of an organism where functioning of the cell is maintained.