An important characteristic of the mushroom cultivation is the "activity" of substrate. The correlations were determined between climate factors and CO2 production per phase of seven crops. The CO2 production was used as a measure for substrate activity. During the vegetative phases high correlations were found between compost temperature, vapour pressure and CO2 pressure in the compost and average CO2 production per phase. In all vegetative phases, average temperature, CO2 pressure and vapour pressure differences between compost and growing room air were highly correlated with the CO2 production. This correlation was also found for differences in vapour pressure during the second and third flush and for CO2 pressure differences between compost and growing room air during the first flush. At equal average temperature, vapour pressure and CO2 pressure in compost, the average CO2 production per phase during the flushes were higher then during the vegetative phases. This might be caused by a larger exchange area due to an increase in the number and size of fruit bodies. Climate factors which were highly correlated with the CO2 production can be used to control crops on the basis of the CO2 production.