In order to assess the usefulness of quantitative in situ rRNA hybridization as an indicator of the physiological state of bacteria, we have used this method to measure the cellular contents of 16 S and 23 S rRNA in Salmonella typhimurium subjected to a number of different stress treatments. The contents of rRNA in S. typhimurium decreased when the bacteria were subjected to carbon starvation, heat stress, and osmotic stress prior to the hybridization, whereas no decrease in the intracellular contents of rRNA was observed when the bacteria were subjected to cold stress, acetic acid or ethanol treatment prior to the hybridization. We must conclude, that the content of 16 S rRNA and 23 S rRNA cannot be used as the sole indicator of the physiological state or viability of food borne pathogens. Viable as well as non-viable food borne bacteria will be detected when methods based on detection of rRNA are used.