A galactocerebroside (BAC-4-4a) was obtained from its parent galactocerebroside molecular species (BAC-4) by using two types of reversed-phase HPLC column. The parent cerebroside BAC-4 has been isolated from less polar lipid fraction of CHCl(3)/MeOH extract of sea cucumber Bohadschia argus, together with known typical types of glucocerebroside molecular species. The structure of BAC-4-4a was determined on the basis of chemical and spectroscopic evidences as 1-O-(beta-D-galactopyranosyl)-(2S,3R,4E)-2[(2'R,15'Z)-2-hydroxytetracosenoylamino]-4-heptadecene-1,3-diol. This is the first report of isolation and structure determination of galactocerebroside from sea cucumber.