We have studied by electron microscopy the organization of microtubules and neurofilaments in axons of giant identified neurons in Aplysia and the effect upon it of microinjected DNase I, a depolymerizer of actin filaments, to determine whether the block of fast transport caused by this agent might be due to disorganization of the cytoskeleton. Microtubules were thoroughly intermingled with the more numerous neurofilaments rather than segregated into domains. In the axon of the cell, GCN, the morphologically identifiable transmitter storage vesicles, which have been shown previously to move by anterograde fast axonal transport, were distributed non-randomly, tending to be near microtubules. They were found most frequently 10-20 nm from the nearest microtubule. When DNase I was microinjected into the axon of the cell R2, large clumps of organelles formed in areas distributed around the perimeter of the axoplasm. Microtubules were abnormally rare in those areas, though abundant elsewhere in the injected region. In some axons injected with DNase I, axoplasm separated from the axolemma at many places around the perimeter of the axoplasm. When the microtubule stabilizing drug taxol was present in the bathing medium, the inhibition of transport caused by injection of DNase I was reduced by about half. We conclude that a substantial part, at least, of this inhibition is caused by localized losses of microtubules. These results suggest that actin is more likely to play a structural role in the axon than a direct role in force generation for fast axonal transport.