Many free-living and parasitic invertebrates have been known to tolerate low oxygen tensions and anaerobic conditions. Von Brand (1946) who reviewed the extensive literature on anaerobiosis in invertebrates listed species of Cyclops which under experimental conditions could tolerate 2–18 h of anaerobiosis. Apart from this, few experimental studies have been made on anaerobiosis in copepods, especially the parasitic species. Availability of a number of live Caligus diaphanus at Plymouth made it possible to make a few preliminary observations on the behaviour of this copepod parasite under anaerobic conditions.