1. The anaerobic coproporphyrinogenase activity in an extract of Rhodopseudomonas spheroides is inhibited by 1,10-phenanthroline, alphaalpha'-bipyridyl, flavins, 2,4-dinitrophenol and 1,4-naphthaquinone. These compounds have no effect on the aerobic coproporphyrinogenase activity. 2. On removal of small-molecular-weight material from a crude extract, the anaerobic system becomes very unstable; it can be stabilized by adding succinate. Now nicotinamide nucleotides, in addition to Mg(2+), ATP and methionine, are required for protoporphyrin to be formed. 3. A mechanism for the anaerobic reaction is proposed, based on the cofactor requirements and the effect of inhibitors. 4. The enzyme responsible for aerobic activity has been partially purified and some of its properties are reported. 5. A crude extract of Chromatium strain D also exhibits coproporphyrinogenase activity under anaerobic conditions in the presence of S-adenosylmethionine or ATP plus methionine. The requirement for other cofactors is variable.