Abstract: Identification of microorganisms based on the sequences of their 5S, 23S and particularly 16S ribosomal RNAs is growing in utility as the database of known ribosomal RNA sequences expands. Experimental identification is usually based on matching the experimentally-determined sequence of an organisms rRNA to a previously-determined sequence in the databank, or hybridization of the organisms rRNA or encoding rDNA to an oligonucleotide probe specific for an organism anticipated to be present in the sample. Here we propose the identification of microorganisms based on the overall composition (not sequence or hybridization propensity) of characteristic molecules derived from their rRNA or rDNA sequences by enzymatic cleavage or localized amplification. Ribonuclease T1 fragments of rRNA composition determination by mass spectrometry are especially favored.