A pigment absorbing at 503 nm (P503) was observed in the difference spectra of several strains of microorganisms. The pigment was present in most facultative anaerobes and absent from many but not all of the aerobes examined. P503 is probably not a component of the normal oxygen-linked electron transport chain since the pigment was present in both respiratory sufficient (ρ⁺) and deficient (ρ⁻) strains of Saccharomyces cerevisiae. There was no apparent correlation between P503 and either the cytochromes or any known metabolic cellular activities. However, the widespread occurrence of P503 in microorganisms suggests that it is of functional significance.