The 3'-terminal sequence of 18S ribosomal RNA from Drosophila melanogaster and Saccharomyces cerevisiae was determined by stepwise degradation from the 3'-terminus and labelling with [(3)H]isoniazid. The sequence G-A-U-C-A-U-U-A(OH) was found at the 3'-terminus of both 18S rRNA species. Less extensive data for 18S RNA from a number of other eukaryotes are consistent with the same 3'-terminal sequence, and an identical sequence has previously been reported for the 3'-end of rabbit reticulocyte 18S rRNA (Hunt, 1970). These results suggest that the base sequence in this region is strongly conserved and may be identical in all eukaryotes. As the 3'-terminal hexanucleotide is complementary to eukaryotic terminator codons we discuss the possibility that the 3'-end of 18S rRNA may have a direct base-pairing role in the termination of protein synthesis.