A rapid procedure has been devised to extract photochemically active reaction centers from the green bacterium Chloroflexus aurantiacus strain J-10-f1 and a mutant lacking colored carotenoids (73-3). The isolated material was completely free of antenna bacteriochlorophyll and cytochromes and nearly free of carotenoids and had near IR absorption maxima at 865, 815, and 756 nm. On oxidation, the peak at 865 nm was bleached and the remaining two peaks were shifted to 806 and 757 nm. Although these spectral characteristics show general similarities to those in reaction center preparations from purple bacteria, there are some distinct differences. Comparison of the spectra of reaction centers of Chloroflexus and Rhodopseudomonas sphaeroides, adjusted to the same absorbance at 865 nm, showed that the absorbance at 815 nm in Chloroflexus was much less and that at 757 nm was much greater than the equivalent absorbances in R. sphaeroides. Unlike reaction centers from R. sphaeroides and other photosynthetic bacteria that have two molecules of bacteriopheophytin and four molecules of bacteriochlorophyll per unit, reaction centers from Chloroflexus appear to have three molecules of each pigment per unit. A prominent shoulder at 793 nm disappears concomitantly with the bleaching at 865 nm on oxidation of Chloroflexus reaction centers; this spectral component may represent the higher energy transition in the near IR of the two bacteriochlorophylls forming P865. While Chloroflexus has a light-harvesting pigment system very similar to that of the Chlorobiaceae, its reaction center has optical absorption characteristics similar to those of the Rhodospirillaceae.