To determine the structure and functional relationships of invertebrate lysozymes, we isolated a new invertebrate (i)-type lysozyme from the common orient clam (Meretrix lusoria) and determined the complete amino acid sequence of two isozymes that differed by one amino acid. The determined sequence showed 65% similarity to a lysozyme from Venerupis philippinarum (Tapes japonica), and it was therefore classified as an i-type lysozyme. The lytic activities of this lysozyme were similar to those of previously reported bivalve i-type lysozymes, but unlike the V. philippinarum lysozyme, it did not exhibit an increase in activity in high ionic strength. Our data suggest that this lysozyme does not have a dimeric structure, due to the replacement of Lys108 which contributes to dimer formation in the V. philippinarum lysozyme. GlcNAc oligomer activities suggested an absence of transglycosylation activity and a higher number of subsites on this enzyme compared with hen egg lysozyme.