Quantitative estimation of the amounts of vitamin B12-active substances in some marine invertebrates was made using a microbiological assay technique. In general, species from phylum Mollusca were found to be richer in these substances than were species from phyla Echinodermata and Annelida and class Crustacea. No definite relationship was found to exist between the content of vitamin B12-active substances in an animal and its phylogeny.