Since the usual measurements of light emission from marine bacteria involve many (10(6)-10(10)) cells, the question has often been raised as to whether or not the individual cell's luminescence is truly continuous. To investigate this question, we assembled a sensitive photo-counting system with computerized data acquisition. Several luminous species were studied: Beneckea harveyi, Photobacterium belozerskii, P. fischeri, and P. leiognathi. Isolated single cells gave count rates ranging from 2 to 10 times the background, depending on the brightness of the strain and the state of induction. No flashes, bursts, or oscillations were evident from data collected in counting intervals of 100 ms, using both photo time-correction and power spectral analysis. Our algorithms could detect an oscillating component with an intensity as low as 0.3% of the average, as determined by the analysis of reference light sources. That photons are emitted randomly was further shown by the fact that the count distribution from the living cell closely matched that of a reference light source attenuated to the same average count rate.