In the plastids of Cuscuta reflexa the number of thylakoids is strongly reduced compared with true chloroplasts and no typical grana are visible. The plastids of Cuscuta europaea lack thylakoids and the stroma is filled with either starch grains or lipid droplets. In Cuscuta reflexa both chlorophylls are present in low concentrations, while in C. europaea chlorophylls are totally absent. Light slightly stimulates the incorporation of (14)CO2 in C. reflexa. This is in accordance with a low activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) which was measured in extracts of this parasite. No stimulation of (14)CO2 incorporation by light was observed in C. europaea. Instead, a relatively strong incorporation was measured in darkness. Malate and aspartate were the main products of this incorporation. In agreement with these results, no Rubisco activity was detected in C. europaea but there was a moderate activity of phosphoenolpyruvate-carboxylase. The presence of the genes for both subunits of Rubisco (rbcL, rbcS) and of the gene which codes for the 32-kDa protein of photosystem II (psbA) was established for C. europaea by hybridization experiments. In both species only very small amounts of transcripts of these genes were detected.