We have used molecular characteristics to clarify the taxonomic status of three problematic gracilarioid algae, by comparison with existing data for the Gracilariaceae. Nucleotide sequences of nuclear-encoded 18S rDNA were determined for an unidentified species of Gracilariopsis from North Carolina, and two infertile strains of gracilarioid algae from Namibia, tentatively assigned to Gracilariopsis. The sequence from the North Carolinian alga was intermediate between those established previously for Gracilariopsis lemaneiformis and European Gracilariopsis sp., and represents a different, probably undescribed, species. Sequences from the Namibian plants were characteristic of Gracilaria, and very similar to those from Gracilaria verrucosa and two closely related congeners. In parsimony, distance and maximum-likelihood analyses of these sequences and others already published for the Gracilariaceae, the North Carolinian Gracilariopsis sp. consistently grouped with its congeners, and the two Namibian strains with verrucosa-type species of Gracilaria. Restriction digestion of plastid DNA from the Namibian algae suggested that they were conspecific despite their different morphologies and agar characteristics. Moreover, the restriction patterns were nearly identical with those observed previously for Gracilaria verrucosa, and the Namibian strains are probably referable to this species.