Thermophilic soil geobacilli isolated from cool temperate geographical zone environments have been shown to be metabolically inactive under aerobic conditions at ambient temperatures (-5 to 25 degrees C). It is now confirmed that a similar situation exists for their anaerobic denitrification activity. It is necessary therefore to determine the mechanisms that sustain the observed significant viable populations in these soils. Population analysis of thermophiles in rainwater and air samples has shown different species compositions which support the view that long distance global transport and deposition in rainwater is a possible source of replenishment of the soil thermophile populations. Survival experiments using a representative Geobacillus isolate have indicated that while cells lose viability rapidly at most temperatures, populations can increase only when the temperature allows growth to take place at a rate which exceeds death rate. Long term (9-month) experiments at 4 degrees C show population increases which can be accounted for by very slow growth rates complemented by negligible death rates. These results are interpreted in the context of current hypotheses on the biogeography patterns of bacteria.