Exposure to cadmium induced the nematode Caenorhabditis elegans to synthesize metallothioneinlike proteins, which were isolated and characterized. The supernatant obtained after disruption of cadmium-exposed nematodes was applied to a Sephadex G-75 column, and cadmium was detected in a high-molecular-weight fraction and a lowmolecular-weight fraction, of which the latter corresponded in elution volume to the well-characterized metallothionein. From this low molecular weight cadmium-rich fraction, two major cadmium-binding proteins were separated by DEAE Sephadex A-25 ion exchange chromatography. These two isoproteins showed similar ultraviolet absorption spectra to mammalian metallothioneins, indicating that they can be categorized metallothioneins. The slower-eluted isoform, MT-II, was as homogeneous on polyacrylamide disc gel electrophoresis while the faster-eluted isoform, MT-I, contained some impurity. The amino acid composition of the purified MT-II was determined. The content of cysteic residues was 22.3 % and the next most abundant residues were aspartate, alanine, glutamate and lysine. The content of acidic amino acids was high and that of aromatic amino acids was very small. The properties were similar to those of metallothioneins found in lower phyla of the animal kingdom rather than in mammalian species.