The polio type 1 (Mahoney) RNA sequence (1) has been analyzed in terms of the distribution of its mononucleotides, dinucleotides and trinucleotides (codons). The distribution of adenosine in the sequence is nonuniform, being lower at the 5' end and higher at the 3' end. The dinucleotide CG is relatively rare and the dinucleotides UG and CA are relatively more common than expected. Codon usage is decidedly nonrandom. Codons containing CG are avoided and those ending in adenosine are favored. The asymmetric use of mononucleotides, dinucleotides and codons in polio RNA is unexplained at the present time although the lowered CG frequency may be the result of a DNA origin for polio RNA.