Choreocolax polysiphoniae Reinsch is a minute, cushion-shaped, variably-pigmented red alga forming a‘parasitic’association with Polysiphoma lanosa. Cortical cells of Choreocolax contain starch grains and small, poorly-differentiated plastids. Through 14C labelling experiments it has been shown that 14C fixed by Polysiphonia as sodium mannoglycerate may be transferred to Choreocolax, although clear differentiation between host and parasite tissues is difficult due to displacement of some host cells within the thallus of the parasite. However, Choreocolax can also fix CO2 photosynthetically, an ability which appears to be partially repressed in the attached state since CO2 fixation progressively increases with time if Choreocolax plants are detached from the host. The products of Choreocolax photosynthesis may include mannitol and an unidentified complex carbohydrate.