The seaweed Fucus vesiculosus is a member of the brown algae family. Kille and co-workers [Biochem. J. 338 (1999) 553] reported that this species contains the gene for metallothionein. Metallothionein is a metalloprotein having low molecular weight, and high cysteine content, which binds a range of metals. F. vesiculosus bioaccumulates arsenic from the aquatic environment [Mar. Chem. 18 (1986) 321]. In this paper we describe arsenic binding to F. vesiculosus metallothionein, characterized by electrospray ionization mass spectrometry. Five arsenic-MT species were detected with increasing As to protein ratios. These results provide important information about the metal-chelation behaviour of this novel algal metallothionein which is a putative model for arsenic binding to F. vesiculosus in vivo.