We analyze the known long natural nucleotide sequences, looking for possible homonucleotide clustering. We find strong evidence for adenine clustering starting at the A3 level. Single, isolated As and to a lesser extent AAs occur less frequently than expected from the base composition of the given sequence alone. A3, A4 and higher A—runs occur much more frequently than. expected. The effect is quite universal and occurs with equal strength in prokaryotes and eukaryotes. Analogues thymine clustering is observed in a single genome only. Cytosines and/or guanines mildly display the reverse trend.