Arsenic compounds were extracted from tissues of various marine animals. Arsenobetaine found in all the animals was almost always the most abundant arsenic compound in the extract. This result showed that arsenobetaine is present in marine animals independent of their feeding habits and trophic levels. Microbial conversion behaviour was investigated with several standard arsenicals. As typical origin of marine microorganisms, sediments, micro-algae, mollusk intestine and suspended substances were used. The degradation of arsenobetaine was observed with every origin, suggesting the ubiquitious occurrence of arsenobetaine-decomposing microorganisms in marine environment. Especially, the microorganisms occurring in the sediments and the suspended substances completely degraded it to inorganic arsenic. From these results it was hypothesized that there is an arsenic cycle that begins with the methylation of inorganic arsenic on the route to arsenobetaine and terminates with the complete degradation of arsenobetaine to inorganic arsenic.