The study describes the development of polymerase chain reaction (PCR) conditions for amplifying eukaryotic small subunit ribosomal (16S-like) rRNA genes from genomic DNA or recombinant plasmids. Oligodeoxynucleotides complementary to conserved regions at the 5' and 3' ends of eukaryotic 16S-like rRNAs were used as primers for DNA synthesis in repetitive cycles of denaturation, annealing, and extension. The fidelity of the amplification products was evaluated by comparing them with previously reported rRNA gene sequences or primer extension analyses. Fewer than one error per 2000 positions were observed in the amplified rRNA coding regions. 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. The study highlights the importance of rRNA sequence analysis in understanding molecular and cellular evolution. It also discusses the challenges of analyzing rRNA sequences from organisms like S. costatum, which is a significant component of coastal phytoplankton communities. The use of PCR methods allowed for the amplification of rRNA genes from small quantities of genomic DNA, making it possible to study rDNA genes from organisms that are difficult to propagate in the laboratory. The results show that PCR amplification of rRNA genes is highly accurate and can be used to determine the phylogenetic relationships between different eukaryotic organisms. The study also demonstrates that the 16S-like rRNA sequence of S. costatum is most similar to those of Achlya bisexualis and Ochromonas danica. The findings contribute to the understanding of the phylogenetic status of S. costatum and its placement within the kingdom Chromista. The study concludes that PCR methods provide a reliable and efficient way to analyze rDNA genes and determine the phylogenetic relationships between different eukaryotic organisms.The study describes the development of polymerase chain reaction (PCR) conditions for amplifying eukaryotic small subunit ribosomal (16S-like) rRNA genes from genomic DNA or recombinant plasmids. Oligodeoxynucleotides complementary to conserved regions at the 5' and 3' ends of eukaryotic 16S-like rRNAs were used as primers for DNA synthesis in repetitive cycles of denaturation, annealing, and extension. The fidelity of the amplification products was evaluated by comparing them with previously reported rRNA gene sequences or primer extension analyses. Fewer than one error per 2000 positions were observed in the amplified rRNA coding regions. 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. The study highlights the importance of rRNA sequence analysis in understanding molecular and cellular evolution. It also discusses the challenges of analyzing rRNA sequences from organisms like S. costatum, which is a significant component of coastal phytoplankton communities. The use of PCR methods allowed for the amplification of rRNA genes from small quantities of genomic DNA, making it possible to study rDNA genes from organisms that are difficult to propagate in the laboratory. The results show that PCR amplification of rRNA genes is highly accurate and can be used to determine the phylogenetic relationships between different eukaryotic organisms. The study also demonstrates that the 16S-like rRNA sequence of S. costatum is most similar to those of Achlya bisexualis and Ochromonas danica. The findings contribute to the understanding of the phylogenetic status of S. costatum and its placement within the kingdom Chromista. The study concludes that PCR methods provide a reliable and efficient way to analyze rDNA genes and determine the phylogenetic relationships between different eukaryotic organisms.