The temporal and depth variability of stimulated bioluminescence at 59 ¿N, 21 ¿W in the North Atlantic during 1991 was measured by two bathyphotometers (BP's). A moored BP (MOORDEX) obtained a 106-day time series of bioluminescence at 50 m between May 1 and August 15. A profiling BP (high-intake, defined excitation, HIDEX) measured bioluminescence in the upper water column during cruises in May and August. MOORDEX-measured bioluminescence intensity cycled with an approximate 20-day periodicity. In May, bioluminescence was correlated with chlorophyll fluorescence with similar vertical distributions in the water column. From May to August, the mean intensity and duration of individual flash events increased, and integrated water column bioluminescence increased by a factor of 4, to 1.2¿1015 quanta m-2. In August, chlorophyll fluorescence distributions and mixed layer depths were shallower than in May. Despite this, the vertical distribution of bioluminescence remained unchanged from May. The following result, potentially caused by typical seasonal changes superimposed upon short-term high variability during each cruise, suggest a change in organism assemblage between May and August: (1) increase in the intensity of flash events, (2) shift to a broader distribution of flash kinetics, and (3) loss of correlation between fluorescence and bioluminescence. ¿ American Geophysical Union 1995