This study was performed to determine the optimal concentration of carbon dioxide, which effects mushroom growth and yield. It was shown that the periods for fruiting initiation, growth and harvest of Flammulina velutipes were increased when the CO2 concentration was raised. In general, those characteristics were less affected in brown strains than in white ones. Especially brown strain ASI4103 was susceptible to changes in CO2 concentration. Yields per bottle and individual mushroom weight also decreased in most strains when CO2 levels increased. We were unable to designate any tendency in the number of fruiting bodies due to the large variation within each respective strain. Finally, water contents in the fruiting bodies were found to decline under high CO2 concentrations.