Summary.The effect of carbon dioxide on tomato plants growing in a small greenhouse has been studied, the gas being produced by burning paraffin in a pressure burner.An increased yield of fruit of 23.9 per cent. for the first half of the season and 13.9 per cent. for the whole season was obtained.A significant increase in the growth‐rate of tomato seedlings was brought about by the treatment.The distribution of carbon dioxide in the glasshouse was investigated and the sources of loss of gas examined.The economic bearing of the results is briefly discusse