Polymerase chain reaction conditions were established for the in vitro amplification of eukaryotic small subunit ribosomal (16S-like) rRNA genes. Coding regions from algae, fungi, and protozoa were amplified from nanogram quantities of genomic DNA or recombinant plasmids containing rDNA genes. Oligodeoxynucleotides that are complementary to conserved regions at the 5' and 3' termini of eukaryotic 16S-like rRNAs were used to prime DNA synthesis in repetitive cycles of denaturation, reannealing, and DNA synthesis. The fidelity of synthesis for the amplification products was evaluated by comparisons with sequences of previously reported rRNA genes or with primer extension analyses of rRNAs. Fewer than one error per 2000 positions were observed in the amplified rRNA coding region sequences. The primary structure of the 16S-like rRNA from the marine diatom, Skeletonema costatum, was inferred from the sequence of its in vitro amplified coding region.