Chiamydomonas in the resting phase of growth has an equal capacity of about 15 micromole 02 uptake per hour per milligram of chlorophyll for both the cytochrome c, CN-sensitive respiration, and for the altemative, salicylhydroxamic acid-sensitive respiration. Altemative respiration capacity was measured as salicylhydroxamic acid inhibited 02 uptake in the presence of CN, and cytochrome c respiration capacity as CN inhibition of 02 uptake in the presence of salicylhydroxamic acid. Measured total respiration was considerably less than the combined capacities for respiration. During the log phase of growth on high (2-5%) C02, the altemative respiration capacity decreased about 90% but retumed as the culture entered the lag phase. When the altemative oxidase capacity was low, addition of salicylic acid or cyanide induced its reappearance. When cells were grown on low (airlevel) C02, which induced a CO2 concentrating mechanism, the altemative oxidase capacity did not decrease during the growth phase. Attempts to measure in vivo distribution of respiration between the two pathways with either CN or salicylhydroxamic acid alone were inconclusive. is activated. In unicellular algae, such as Chlamydomonas, Dunaliella, Chlorella, and Euglena, the oxidation ofglycolate formed during photosynthesis is catalyzed by a mitochondrial membrane bound glycolate dehydrogenase which also oxidizes i-lactate (24). This dehydrogenase does not transfer electrons directly to oxygen as does the FMN-linked, peroxisomal glycolate oxidase. By using a cytochrome oxidasedeficient mutant of Chlamydomonas reinhardtii, Husic andTolbert (1 1) showed that glycolate and D-lactate metabolism could proceed through a SHAM-sensitive, alternative, electron transport pathway. These considerations raised the question whether there might be a difference in the cyanidesensitive (Cyt c) and cyanide-insensitive (alternative) respiration by C. reinhardtii during their growth cycle in low C02 (air) or high C02.