From enrichment cultures of photosynthetic sulphur bacteria, a Chlorella was obtained which combines capacities both for normal photosynthesis in a carbonate medium and for the utilization of methane as carbon source for its growth. This alga is adapted to anaerobic conditions, but the photo-synthetic oxygen production enables the alga itself to affect the environment in this respect. The first algal cultures in liquid medium contained quite a number of bacteria which could be removed by plating on the same medium solidified with agar and with 1% glucose added. The remaining contamination of bacteria amounted to one bacterium per 104 algal cells. As the bacterial growth in batchwise experiments was insignificant, the methane consumption of the alga cannot be disputed. Thus, oxidation of methane to carbon dioxide by Pseudomonas methanica which might subsequently contribute to the growth of the alga did not occur, nor was it possible to indicate methane oxidizing bacteria on media specially suited for this purpose. This alga may be of industrial importance for the production of cell mass on methane-carbonate basis, even without a supply of free oxygen. It could also be used in a closed system in order to remove CO2 and CH4 from the atmosphere and simultaneously bring about an oxygen enrichment. The existence of methane-consuming algae may indicate that complete photosynthesis has occurred on Earth earlier than supposed and also that such photo-synthesis is conceivable on planets with reducing atmosphere, assuming that the conditions in other respects would permit it.