One of the most remarkable features of the brain is the degree of precision with which neurons are interconnected. As the neural circuitry of the nematode Caenorhabditis elegans has been completely established through serial section electron micrographs, and because the nematode is amenable to genetic analysis, it is an excellent system to study the genetic basis of axonal outgrowth and process placement in the formation of a neural network. Here, I will describe the identification of 11 genes which affect the axonal outgrowth and guidance of five pairs of lumbar neurons in C. elegans. A pair of bilaterally symmetric luumbar ganglia are situated in the tail of C. elegans, each consisting of 12 neurons. We have examined the morphology and pattern of axonal processes of three embryonic (PHA, PHB, and PLM) and two postembryonic (PHC and PVN) lumbar neurons on the wild type and existing uncoordinated (unc) mutants, immunocytochemically. We have earlier shown that antibodies against horseradish peroxidase (HRP) stain PHA, PHB, PHC and PVN neurons, and monoclonal antibodies specific to different tubulin isotypes stain the mechanosensory neuron PLM. A monoclonal antibody TY21, raised against C. elegans crude homogenate is specific for phasmid neurons PHA, and PHB. In the wild type, PHA, PHB, PHC, and PVN send anteriorly directed processes ventrally via the lumbar commissures to targets in the ventral cord; whereas, PLM sends a process anteriorly in a subventral position, sending a branch ventrally to the ventral cord near the vulva. PHA, PHN, PHC, and PLM are bipolar neurons, extend a backward process into the tail spike. Mutations in nine genes (
unc-6,
unc-13,
unc-33,
unc-44,
unc-51,
unc-61,
unc-71,
unc-73, and
unc-98) result in abnormal axonal outgrowth and process placement of PHC, PCN and PLM neurons. Four (
unc-6,
unc-33,
unc-44, and
unc-51) of the nine genes identified above, and
unc-76 were previously shown to affect the growth of PHA and PHB neurons. Mutants in
unc-53, which are apparently normal in the growth of PHA, PHB, PHC, and PVN neurons, and
unc-76, harbor defects in PLM axonal outgrowth and guidance. In summary, we have identified a set of 11 genes, which exert overlapping, but distinct effects on the axonal outgrowth and process placement of embryonic (PHA, PHB, and PLM), and postembryonic (PHC and