A commercially available DNA probe (the AccuProbe Mycobacterium kansasii culture identification test, Gen-Probe, USA) for the identification of Mycobacterium kansasii was tested on a panel of 143 fully characterized mycobacterial strains. The isolates included 70 Mycobacterium kansasii and 73 mycobacteria other than kansasii. The specificity was 100% while the sensitivity was 72.8%. This sensitivity is unusually low in comparison with that of commercial DNA probes for other mycobacteria and confirms a previous study that found genetic heterogeneity within the species Mycobacterium kansasii. Strains that do not hybridize with the AccuProbe are particularly prevalent in Italy and perhaps elsewhere in Europe.