We recently reported (Gueneau et al., 1998) the existence of a GC-rich region downstream of the trnR gene which was conserved in the plastidial genome of three algae: Thalassiosira weissflogii, Odontella sinensis (Bacillariophyceae) and Porphyra purpurea (Rhodophyceae). We identified this GC-rich region as a possible vestigial ORF and noted additional high similarity to an open reading frame (ORF) in a plastid from a fourth alga, Pavlova lutherii (Haptophyceae); sequence data from P. lutherii did not extend to the TΨC tRNA arm. We now report that this region instead encodes a recently recognized type of RNA known as tmRNA (Fig. 1A).