The distribution of microtubules and mitochondria in central axons of an insect (Periplaneta americana) is assessed by comparison between counts on micrographs and computed axon random ‘models’. These studies show that the observed multiple association of microtubules with individual mitochondria is statistically highly significant. Electron micrographs of thin sections show that linkage is effected by physical cross-bridge, possibly comprising components from the microtubule and mitochondrion. Linear particle arrays are described on the outer mitochondrial membrane in freeze-fracture replicas, and tentatively related to the bridges seen in thin sections. The results are discussed in terms of proposed roles of microtubules in neurons and other cells.