The hereditary spastic paraplegias (SPG1-29) comprise a group of inherited neurological disorders characterized principally by spastic lower extremity weakness due to a length-dependent, retrograde axonopathy of corticospinal motor neurons. Mutations in the gene encoding the dynamin superfamily member atlastin-1, an oligomeric GTPase highly localized to the Golgi apparatus in the adult brain, are responsible for SPG3A, a common autosomal dominant hereditary spastic paraplegia. A distinguishing feature of SPG3A is its frequent very early onset, raising the possibility that developmental abnormalities may be involved in its pathogenesis. Here we demonstrate that several missense SPG3A mutant atlastin-1 proteins have impaired GTPase activity and thus may act in a dominant-negative, loss-of-function manner by forming mixed oligomers with wild-type atlastin-1. Using confocal and electron microscopy we have also found that atlastin-1 is highly enriched in vesicular structures within axonal growth cones and varicosities as well as at axonal branch points in cultured cerebral cortical neurons, prefiguring a functional role for atlastin-1 in axonal development. Indeed, knock down of atlastin-1 expression in these neurons using small hairpin RNAs reduces the number of neuronal processes and impairs axon formation and elongation during development. Thus, the "long axonopathy" in early-onset SPG3A may result from abnormal development of axons due to loss of atlastin-1 function.