The nucleotide sequences of the duplicate PgiC genes that encode the cytosolic iso- zymes of phosphoglucose isomerase (EC 5.3.1.9) in Clarkia xantiana are described and compared to the previously sequenced duplicate PgiCl and PgiC2 genes of C. lewisii, a species assigned to a different section. The genes of C. xantiana have the same structure of 23 exons and 22 introns in positions exactly corresponding to those in C. lewisii. The exons of the two genes in C. xantiana are highly similar with 96.3% nucleotide identity; the introns show 88.9% identity. However, each of the genes in C. xantiana is even more similar to one of the PgiC genes in C. lewisii with between- species comparisons of corresponding genes showing about half as many nucleotide and amino acid substitutions as the within-species comparisons. The two genes of C. xantiana are therefore designated PgiCl and PgiC2 according to their resemblance to the genes of C. lewisii. All pairwise comparisons of nucleotides, amino acids, and insertions/deletions, as well as DNA parsimony, maximum likelihood, and Fitch-Margoliash analyses, group the PgiCl genes, one from each species, separately from the PgiC2 genes. Thus, the same gene duplication accounts for the presence of two isozymes in each of the species and, by inference, in all the species of the two sections of Clarkia they represent.