Oxyrrhis marina Dujardin is a non-photosynthetic dinoflagellate whose obvious mode of nutrition is phagotrophy. The method of feeding was described by Barker (1935). It is an extremely euryhaline and hardy organism and is apparently very successful in brackish habitats such as supralittoral rock pools and ditches (Droop, 1953 a). Owing to the ease with which it can be cultivated Oxyrrhis is a choice for the initial study of phagotrophy among the predominantly plant-like Dinoflagellata.