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WE have studied, by immunofluorescence, the organization of microtubules in growth cones in dissociated neuronal cell cultures. Growth cones have a large pool of soluble tubulin and in conventionally fixed cultures the entire growth cone, including the filopodia, is labelled with tubulin antibodies, thus obscuring the microtubules. In contrast, in cultures fixed with fixatives containing detergent, the soluble pool of tubulin is removed allowing clear views of the microtubules. On entering the growth cone from the neurite, microtubules splay out and may extend as far as the base of filopodia. In most growth cones, one or more individual microtubules extend into filopodia, aligning with the parallel bundle of microfilaments. This observation suggests one way growth cone motility and neurite advance may be coupled.