Summary

1. Increased CO2 concentration in the air has a positive effect on mycelium growth in the cultivation of Pleurotus ostreatus, P. florida and P. eryngii (Agar cultures). 2. Between 16 % and 22 % CO2 in the air is the optimum for the growth of the 3 above-mentioned Pleurotus species. 3. Concentrations of 36 vol. % CO2 inhibited growth of all 3 species. 4. Both sterile and pasteurized straw substrata showed 20-25 % (of vol.) CO2 3 days after inoculation. Mycelium growth was normal. CO2 concen-tration stands in relation to the amount of inoculation material, tem-perature, composition of substratum, and other environmental factors. 5. The stimulating effect of CO2 on the growth of the Pleurotus species studied and their high tolerance to CO2 have a special importance in Pleurotus farming. 6. Under semianaerobic conditions with high CO2 concentration in the gaseous phase of substratiim, the competitive microorganisms are eli-minated and Pleurotus mycelium can grow well in the non-sterile s\ib-stratum. Carbon dioxide belongs to the most important ecological factors. The quantity of CO2 in the ecosystem determines bio-logical activity and the quantity of micro-organism (Ahrens & Thalmann, 1970; Zadrazil, 1971, etc.). On the contrary one can consider the differing concentrations of CO2 in the eco-system as a selective factor in the range of species of micro-organisms (MacFadyen, 1973; Zadrazil, 1973a). The CO2 requirement of bacteria, yeasts and moulds has lon