We investigated mixed-layer bioluminescence from early April to late September (in April 1989, May 1991, July 1983 and 1990, August 1991, September 1988 and 1989) at stations near the Marine-Light--Mixed Layers (MLML) bio-optical moorings site. Volume-specific bioluminescence potential (BPOT, photons per unit volume) from epipelagic organisms was estimated directly with a pump-through bioluminescence photometer (BP) in 1983, 1988, and 1991. For all cruises, BPOT was also estimated by summing for a volume of seawater, the measurements of each species' total stimulable bioluminescence multiplied by each species' numerical abundance in the volume. The abundance data were taken from bottle casts, net tows, and BP effluent nets. After the onset of the spring bloom, from May through September, mixed layer BPOT was fairly constant, ~1--4¿1014 photons m-3. On one early April cruise (1989) before the spring bloom, BPOT was two orders of magnitude lower. Heterotrophic dinoflagellates in the genus Protoperidinium generally produced most (90% or more) of the mixed layer BPOT in the spring, summer, and fall. On one cruise in September (1988), the autotrophic dinoflagellate Ceratium fusus produced the bulk of the mixed layer BPOT (more than about 4¿1014 photons m-3). Other autotrophic dinoflagellates in the genus Gonyaulax and mesozooplankton produced a minor part of BPOT at most stations. The relative contribution of all autotrophic dinoflagellates to BPOT increase from a few percent during the May--June--July period to ~10% during the August--September period. In situ mechanically stimulable bioluminescence was reduced when underwater scalar irradiance (wavelengths 400--700 nm) was greater than 0.1 μmol photons m-2 s-1. ¿ American Geophysical Union 1995