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Distinctive properties are identified in the molecular structure of ribulose, 1,5-bisphosphate carboxylase/oxygenase (RuBPCase) in chlorophyll c-containing alpe (i.e., chromophytes). Using purified enzyme from Cryptomonas sp., Coccolithophora sp., and Cylindrothecafusiformis, we have determined that the RuBPCase holoenzyme of each species has a molecular weight, subunit composition, and isoelectric points of its subunits similar to the purified enzymes from pea and Chlamydomonas reinhardtii. The large subunits from chromophytes exhibit microheterogeneity in their isoelectric points, whereas two to four well-resolved isoelectric variants of the small subunit were observed in each RuBPCase preparation. In spite of the high degree of similarity in terms of physical properties, both the small and large RuBPCase subunits of the chromophytes are structurally different from those of chlorophytes; immunological studies demonstrate that RuBPCase subunits of these two groups have few antigenic determinants in common.Ribulose 1,5-bisphosphate carboxylase/oxygenase is a bifunctional enzyme which initiates the photosynthetic reduction of CO2 and the first step of the photorespiratory pathway. Present in all autotrophs, RuBPCase3 is probably the most abundant enzyme in nature. In vascular plants, green algae, and most bacteria, the RuBPCase holoenzyme is a high mol wt complex (-550 kD) composed of eight copies each of large (-55 kD) and small (-15 kD)