Caenorhabditis elegans has two genes, unc-59 and unc-61, encoding septin-family GTPases. Mutations in the septin genes cause defects in locomotory behavior that have been previously attributed to cytokinesis failures in postembryonic neuroblasts. We find that mutations in either septin gene frequently cause uncoordination in newly hatched larvae in the absence of cytokinesis failures. The septins exhibit developmentally regulated expression, including expression in various neurons at times when processes are extending and synapses are forming. Motor neurons in the mutant larvae display defects in multiple aspects of axonal migration and guidance that are likely to be responsible for the locomotory behavior defects. The septins are also expressed in migrating distal tip cells, which are leaders for gonad arm extension. Septin mutants affect morphology of the distal tip cells, as well as their migration and guidance during gonadogenesis. These results suggest that septins may be generally required for developmental migrations and pathfinding.