Summary1The effect on the tomato crop of atmospheres enriched in respect of carbon dioxide liberated by two methods with possibilities of commercial application, (1) a fermentation process, and (2) a portable stove, is compared with that of carbon dioxide liberated by acid from sodium bicarbonate.2The concentration of carbon dioxide obtained from the fermentation process was insufficient to give any increase in yield. Using the stove method, which gave a concentration of at least 18 parts per 10,000 for 1.5 hours twice daily, the average increase in crop was 16 per cent. where plants were treated throughout the season and was 9 per cent. where they were treated for the first half of the season only.3Relatively high concentrations of carbon dioxide are shown to be normally present in cucumber houses. It is suggested that, under the weak light conditions prevailing in such houses, the concentration is sufficient for the requirements of the crop.4Mechanical mixing of the carbon dioxide and air is not essential from the standpoint of crop production.5In the case of tomatoes grown in enriched atmospheres, the cultivation of two crops, planted January and June respectively, was not profitable under the experimental conditions.6The stove method of treatment is discussed in relation to commercial practice.