The principal findings of this report on the morphology and electrophysiology of the possum optic nerve are: (i) There are about 230,000 fibres in the optic nerve. This fibre count, based on electron microscopy, is slightly less than a previously reported estimate of the total number of ganglion cells in the possum retina. (ii) The majority (greater than 98%) of the fibres of the optic nerve are myelinated axons of retinal ganglion cells. The diameters of these fibres range from 0.4--4.6 micrometer (axon diameter range: 0.3--3.8 micrometer) and the frequency distribution of the fibre diameters (and axon diameters) is positively skewed and unimodal. (iii) The antidromic compound action potential of the possum optic nerve shows four negative peaks following stimulation of the optic chiasm. These peaks are associated with four conduction latency groups of fibres which have been designated t1, t2, t3 and t4 in order of increasing conduction latency. (iv) The mean peak conduction velocities of the fibres in the conduction latency groups are 13.1 ms-1 (t1), 8.1 ms-1 (t2), 5.7 ms-1 (t3) and 3.1 ms-1 (t4). (v) There is no direct correlation between the frequency distribution of fibre (or axon) diameters as measured by electron microscopy of transverse sections of fixed optic nerve and the conduction latency groups. (vi) The reconstruction of the possum optic nerve compund action potential on the basis of either axon or fibre diameter frequency distribution does not provide an acceptable, indirect correlation between the morphology and the electrophysiology of this optic nerve.