The penetration of light into a photobioreactor and its relation to hydrogen production were analyzed using a photosynthetic bacterium, Rhodobacter sphaeroides. A photobioreactor composed of four compartments aligned along the light penetration axis, each with a 0.5-cm light path, was used to examine the hydrogen evolution at various light penetration depths. The light energy decreased quasi-exponentially upon passage through the bacterial suspension (1.5 mg dry wt./ml). In the first compartment (0-0.5 cm), 69% of the incident light energy was absorbed, 21% in the second one (0.5-1.0 cm) and 7% in the third one (1.0-1.5 cm). However, the hydrogen evolution rates did not decrease as the light energy. The efficiency of the conversion of light to hydrogen increased with the depth in the reactor and the third compartment showed the highest efficiency. Excess absorption of light energy in the shallow region reduced the total efficiency of the reactor. Alteration of the light spectrum upon passage of the light through the bacterial suspension greatly affected the hydrogen production. In the deep region of the reactor, energy of light of wavelength around the absorption maxima of the bacterium (800-850 nm) was lost. Light reaching the deep region was mainly of wavelength between 600 and 780 nm, and was also used for hydrogen production.