We investigated the accumulation, distribution, and elimination of dietary arsenobetaine in Atlantic salmon and Atlantic cod, with the focus on muscle, liver, and kidney tissues. The fish were exposed to dietary arsenobetaine for three months, followed by a three-month depuration period. The two species showed marked differences in the accumulation and elimination of arsenobetaine. Total arsenic concentrations in Atlantic salmon increased significantly in muscle, liver, and kidney, whereas in Atlantic cod, a significant increase in total arsenic concentration was observed only in muscle. Elimination kinetics in muscle were distinct between the two species, with elimination half-lives from muscle tissue estimated at approximately 77 d in Atlantic cod and 37 d in Atlantic salmon, resulting in an absorption efficiency approximately twofold higher in Atlantic cod compared to that in Atlantic salmon. Provision of the universal methyl donor S-adenosylmethionine to bioconversion mixtures promoted both the rate and extent of arsenobetaine formation. These findings suggest that in the proposed biosynthesis of arsenobetaine from dimethylarsinoylethanol, oxidation would precede the reduction and methylation at the arsenic atom. The presence of enzyme capable of methylating dimethylarsinoyl acetate in a bacterial isolate from marine mussel, highlights a possible direct involvement of prokaryotic organisms in the biosynthesis of organoarsenic compounds within marine animals.