Bacteriochlorophyll (Bchl) a was found in strains of the strictly aerobic acidophilic heterotrophic bacteria: Acidiphilium rubrum, A. angustum, and A. cryptum. Absorption spectra of the cell-free extracts showed a large peak at 865 nm and a small peak around 802 nm. Anaerobic growth was not observed in either the light or the dark. Bchl a was not detected in either A. organovorum or A. facilis. Bchl a contents were less than ca. 0.7 nmol/mg dry cell weight, being variable among the species.