Nucleotide (nt) sequences of the nuclear-encoded small-subunit (18S) rRNA genes are compared in representatives of the non-parasitic genera of the marine red algal order Gracilariales. Sequence divergence within the order is relatively large and thus may have taxonomic utility. Differences in 45 or more nt positions (substitutions, insertions/deletions) were found between most genera. Within Gracilaria, subgenera differed by 12–24 nt, and on this basis it is suggested that Hydropuntia may be a subgenus of Gracilaria, and that G. chilensis Bird, McLachlan et Oliveira does not belong to subgenus Textoriella but represents another subgenus. By contrast, European and Pacific members of Gracilariopsis differedby 56–60 nt, suggesting that this is a relatively old genus of conservative morphology. In parsimony, distance, and maximum-likelihood trees based on the sequence data, Gracilariopsis was well separated from the rest of the order, the austral genera Curdiea and Melanthalia formed a clade broadly ...