AbstractGas production byChlamydomonas moewusii inthe light has been followed by manometric techniques during the adaptation to anaerobiosis. The only detectable gases produced are CO2and H2CO2is produced at a rather constant rate whereas H2evolution increase with time. This increase of H2evolution during the adaptation period can be inhibited by cycloheximide and by chloral hydrate, two inhibitors of protein synthesis. If the inhibitors are added to already adapted cells there is no effect on H2evolution. Adapted cell suspensions are sensitive to oxygen. Incubation under O2for 10 min inhibits the H2evolution to 100%. After removal of oxygen the capability to evolve H2can be restored only by a new adaptation period. This second adaptation to H2evolution can also be inhibited by cycloheximide.