Bacteriochlorophyll alpha-protein from Prosthecochloris aestuarii strain 2K was oriented in a pulsed electric field. The room temperature linear dichroism spectrum of the oriented protein in the Qy region of the bacteriochlorophyll alpha absorption exhibits a single asymmetrical peak at 813 nm with a shoulder extending to the blue. The approximately equal 12 nm fullwidth of the linear dichroism peak is only about half that of the 300 K absorption spectrum. The linear dichroism at 813 nm was not saturated at field strengths of up to 15 kV/cm. The time dependence of the linear dichroism suggests that the orienting particles are aggregates of at least some tens of bacteriochlorophyll alpha-protein trimers. The linear dichroism peak coincides in wavelength with the 813-nm peak of the 300 K, 4th derivative absorption spectrum of the protein and is therefore attributed to the bacteriochlorophyll a Qy exciton transition observed in absorption at the same wavelength.