Caenorhabditis elegans is the first animal whose genomic sequence has been determined. One of the new possibilities in post-sequence genetics is the analysis of complete gene families at once. We studied the family of heterotrimeric G proteins. C. elegans has 20 Galpha, 2 Gbeta and 2 Ggamma genes. There is 1 homologue of each of the 4 mammalian classes of Galpha genes, G(i)/G(o)alpha, G(s)alpha , G(q)alpha and G12alpha, and there are 16 new alpha genes. Although the conserved Galpha subunits are expressed in many neurons and muscle cells, GFP fusions indicate that 14 new Galpha genes are expressed almost exclusively in a small subset of the chemosensory neurons of C. elegans. We generated loss-of-function alleles using target-selected gene inactivation. None of the amphid-expressed genes are essential for viability, and only four show any detectable phenotype (chemotaxis defects), suggesting extensive functional redundancy. On the basis of functional analysis, the 20 genes encoding Galpha proteins can be divided into two groups: those that encode subunits affecting muscle activity (homologues of G(i)/G(o)alpha, G(s)alpha and G(q)), and those (14 new genes) that encode proteins most likely involved in perception.