MicroRNAs (miRNAs) and small-interfering RNAs (siRNAs) are small non-coding RNAs that play important regulatory roles in animals and plants. The primary sequences of 9164 miRNAs and 14238 siRNAs were analyzed to determine the occurrence of each nucleotide in specific positions of the sequences. The results show that there are positions in which the composition is not completely random. In general the nucleotide cytosine is underrepresented in both miRNA and siRNA sequences, while the nucleotides uracil and adenine are overrepresented in miRNAs and siRNAs, respectively. Possible implications between these findings and the biological functions of small non-coding RNAs are discussed.Keywords: microRNA, RNA primary sequence, ur