A wide range of organic compounds that contain atoms of arsenic have been found in fish and other marine organisms, and these include lipids, although these are usually present at low levels. Some authors take a rather wide view of what constitutes an arsenolipid and thus define compounds such as trimethylarsine and its metabolites as lipids, simply based on their solubility in organic solvents. I do not consider this an appropriate definition (see my web page on nomenclature), and only those lipids, both simple and complex, which have been characterized sufficiently to be certain that they contain long aliphatic chains and arsenic atoms somewhere in the molecule are considered here. Methylation of inorganic arsenic may be a de-toxification method in some organisms that yields products that can be incorporated into organic compounds including lipids with fish at the end of the marine food chain, but the biosynthetic mechanisms for arsenolipids have not been determined. While most researh has been concerned with marine organisms, arsenolipids can be produced in freshwater algae (e.g. Chlamydomonas reinhardtii) grown in a medium containing arsenate. While the potential toxicity of arsenolipids to human consumers of fish products requires further evaluation and monitoring, there appears to be little cause for concern at present. With improvements in analytical methodology, more of these arsenolipids are being identified, characterized and quantified.