The single-copy nuclear gene (GapA), encoding the plastid-localized glyceraldehyde-3-phosphate dehydrogenase (GAPDH) of the marine red alga Gracilaria verrucosa, has been cloned and sequenced. The GapA transcriptional initiation site was located 49 bp upstream of the start codon, and a putative TATA box was found 54 bp farther upstream. A spliceosomal intron was identified in the transit-peptide-encoding region in a position very similar to intron 1 of GapA and GapB of higher plants; no introns occur in the region encoding the mature protein. These observations provisionally suggest that both red algae and higher plants descend from a single ancestral photosynthetic eukaryote, i.e. that a single endosymbiotic event gave rise to red algal and higher-plant plastids.