Lysosome associated membrane proteins (LAMPs) constitute a family of vertebrate proteins located predominantly in lysosomes, with lesser amounts present in endosomes and at the cell surface. Macrosialin/CD68s are similar to LAMPs in their subcellular distribution and amino acid sequence and presumed structure across the carboxyl terminal two thirds of their length. The functions of LAMPs and CD68s are not known. In the present study, a bioinformatics approach was used to identify a Caenorhabditis elegans protein (LMP-1) with sequence and presumed structural similarity to LAMPs and CD68s. LMP-1 appears to be the only membrane protein in C. elegans that carries a GYXX(phi) vertebrate lysosomal targeting sequence at its C terminus (where (phi) is a large, hydrophobic residue). LMP-1 was found to be present from early embryonic stages through adulthood and to be predominantly localized at the periphery of a population of large, membrane-bound organelles, called granules, that are seen throughout the early embryo but in later stages are restricted to the cells of the intestine. Analysis of an LMP-1 deficient C. elegans mutant revealed that LMP-1 is not required for viability under laboratory conditions, but the absence of LMP-1 leads to an alteration in intestinal granule populations, with apparent loss of one type of granule.