In laboratory tests, the spider crabsNotomithrax ursus, collected near Kaikoura peninsula, South Island, New Zealand, were shown to mask with some species of algae more than others. The types of algae most preferred as masking material were branched algae such asHalopteris spicigeraandCorallina officinalis. Thallose‐like algae such asUlvasp., and strap‐like algae such asCladhymenia oblongifolia, were less preferred. It was also shown that algae with whichN. ursusprefers masking are also the algae thatN. ursusprefers to eat. The conflicting requirements of masking and food storage, and possible reasons for algal choice are discussed.