The respiration of Rhodospirillum rubrum has been studied with resting wild-type cells.A few experiments on "blue-green" mutant cells of RhodoDseudomonas soheroides have been performed. With succinate as substrate, rates of oxygen uptake in the dark, and in light at intensities sufficient to saturate photometabolism, are compared. Variables studied have been pH, temperature, physical manipulations (freezing and thawing, degree of anaerobiosis during harvest and storage), and concentration of various respiratory inhibitors and alcohols (narcotics). Light suppression of oxygen uptake can be considered the result of two processes, one of which is light-sensitive. Oxygen uptake in the absence of light ("dark respiration") is the sum of these two processes, "lightsensitive" plus "light-insensitive"; oxygen uptake in light ("light respiration") repre-* No. 170 in the series "Publications of the Giadu-