A method of characterizing an unknown organism species comprises determining the position of part or whole of evolutionarily conserved sequences in genetic material of said organism, relative to a known position of restriction endonuclease cleavage sites in said genetic material (other than by determining the chromatographic pattern of retriction endonuclease digested DNA from said unknown organism, which digested DNA has been hybridized or reassociated with ribosomal RNA information containing nucleic acid from or derived from a known probe organism), thereby to obtain an identifying genetic characterization of said unknown organism, and comparing said characterization with information from at least two sets of identifying genetic characterizations derived from the same conserved sequences, each of said sets defining a known organism species.