Ribulose-1,5-bisphosphate carboxylase (Rubisco) from the algae Olisthodiscus luteus (chromophyte) and Griffithsia pacifica (rhodophyte) are remarkably similar to each other. However, both enzymes differ significantly in the structure and function when compared to Rubisco from green algae and land plants. Analysis of purified Rubisco from 0. Iuteus and G. pacifica indicates that the size of the holoenzyme and stoichiometry of the 55 and 15 kilodalton subunit polypeptides are approximately 550 kilodaltons and eight:eight for both algae. Antigenic determinants are highly conserved between the 0. Iuteus and G. pacifica enzymes and differ from those of the spinach subunit polypeptides. Sequence similarity between the two algal large subunits has been further confirmed by one-dimensional peptide mapping. Substrate ribulose bisphosphate has no effect on the rate of CO2/Mg2+ activation of 0. luteus and G. pacifica enzymes which contrasts to the extensive inhibition of spinach Rubisco activation at similar concentrations of this compound. In addition, the Michaelis constant for CO2 and the inhibition constant for 6-phosphogluconate are similar for the 0. luteus and G. pacifica catalyzed carboxylation reaction. Both values are intermediate to those observed for the tight binding spinach enzyme and weak binding prokaryotic (Rhodospirillum rubrum) enzyme. The biochemical similarities documented between 0. Iuteus and G. pacifica may be due to a common evolutionary origin on the chromophytic and rhodophytic chloroplast but could also result from the fact that both subunit polypeptides are chloroplast DNA encoded in these algal taxa.Rubisco4 is present in all photoautotrophic cells. Information defining the natural variation of Rubisco within diverse plant taxa may provide insight to the molecular evolution of this chloroplast protein component and the structural/functional constraints that govern enzyme action. Unfortunately, although many parameters of Rubisco biochemistry have 'Supported by NSF grant PCM 8402300 to R. A. C. and a NIH predoctoral fellowship (GM07270) to S. M. N.