Genetic and molecular studies have identified two different sodium channel genes in Drosophila, para and DSC1. The functional contributions of the para-encoded channel have been inferred from analysis of mutant phenotypes. However, no mutations of DSC1 have been identified, so the in vivo functions of the channel it encodes are not yet known. To learn more about the possible functions of DSC1 in the Drosophila nervous system compared with those of para, we have characterized the expression patterns of these two sodium channel genes at embryonic, larval, pupal, and adult stages by tissue in situ hybridization, para encodes the predominant type of sodium channel and is ubiquitously transcribed throughout the CNS and PNS at all developmental stages. The expression pattern of DSC1 is very different from para during embryonic and larval stages during which there are very few DSC1-expressing cells in either the CNS or PNS. Double- labeling studies suggest that some of these cells are non-neuronal. However, in pupal and adult stages, para and DSC1 have completely overlapping patterns of expression in the CNS and retina. In the pupal and adult, PNS expression of these genes is still distinct because only para transcripts are detected in wing sensory neurons. The strong and widespread expression of DSC1 in the CNS of pupae and adults suggests that the DSC1 channels are likely to provide an important function in neurons during these stages. Since most, if not all, neurons in the pupal and adult CNS express both para and DSC1, these two sodium channel genes probably subserve distinct functions within these cells. Our results provide the background for elucidating the respective in vivo contributions of para and DSC1 to neuronal excitability and for dissecting the regulatory mechanisms that underlie their different patterns of expression.