The Na+,K+-ATPase activity of membranes from a behavioral mutant of C. elegans was found to be about one-third that of the wild-type. The levels of mRNA and polypeptide of Na+,K+-ATPase alpha-subunit in the mutant were as high as those in the wild-type, but the level of the phosphorylated intermediate of the Na+,K+-ATPase in the mutant worm was 80% lower than that in the wild-type. A single predicted amino acid replacement (Leu to Phe) was found in a highly conserved region in the alpha-subunit that is involved in the formation of phosphorylated intermediate. The abnormal feeding behavior of the mutant worm may be attributed to this missense mutation. (C) 1998 Academic Press.