Computerized methods were used to analyze published sequence information from Rhodospirillum rubrum ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBPCase) and the sequence of the large subunit of spinach RUBPCase. The sequences of 7 peptides from the bacterial enzyme were compared to the entire sequence of the spinach large subunit to find optimal alignments and test statistical significance. Several of the R. rubrum RUBPCase peptides align unambiguously with the spinach sequence, and the alignment of the largest peptide is clearly significant in a statistical sense. The total of 91 positions aligned with R. rubrum peptides includes 30 identities. Our analysis strongly suggests that the entire sequence of the R. rubrum enzyme, when completed, will be found to exhibit statistically significant similarity to the large subunit of plant RUBPCase.