The heterotrophic bacteria population occurring on decaying sandstone from ancient monuments has been examined. Quantitative and qualitative observations are presented for seasonal variation over a period of 1 year. The diversity of heterotrophs occurring on sandstone was assessed on a monthly basis using a range of isolation media. An increase in numbers of bacteria was seen during the winter months; with both Gram-negative and Gram-positive types present. Gram-positive bacteria, especially spore-formers of the genus Bacillus, were found exclusively in the dry summer months. Decay activity, reflecting the potential of natural mixed populations present on the stone, also showed seasonal variation and this was related to the pattern of change for the bacterial species observed.