Polysiphonia lanosa (L.) Tandy is a marine red alga that usually grows epiphytically on the fucale Ascophyllum nodosum (L.) Le Jolis. The present work was conducted in order to obtain more information on the relationships between these two algae, especially as regards the metabolism and long-distance transport of phosphorus. Three types of experiments were carried out using labelled phosphorus. (1) Comparative study of the metabolism of 32P1 absorbed by the tissues of each species. By means of two-dimensional chromatography and autoradiography, it was shown that 32P1 was rapidly incorporated into organic soluble compounds (adenosine triphosphate, hexose monophosphate, uridine diphosphoglucose, phosphoenolpyruvate + phosphoglyceric acid). Although the two algae belong to different phylae the phosphorylated compounds were not very different. The energy charges (0. 72 for both species) were in the usual range for aerobic plant tissues. On the other hand the incorporation of 32P1 into the insoluble P0 fraction was doubled in P. lanosa compared to in A. nodosum (ca 80 and 40%, respectively). At the source level, the air bladder of A. nodosum. the same soluble compounds (inorganic phosphate, P1 adenosine triphosphate, hexose monophosphate. etc.) represented the likely forms transported. A part of the soluble P0 fraction may return to the P1 fraction. (2) In translocation experiments conducted in situ, 32P1 locally injected into an air bladder moved over long distances not only through the thallus of A. nodosum but also into P. lanosa. The reciprocal transfer remained unsuccessful. (3) The 32P1 represented the predominant compound identified in the two species: this argues in favour of P1 as the translocated form of phosphorus. Our results support the hypothesis of a parasitic rather than a simple epiphytic relationship between the two algae.