The underwater light field and phytoplankton abundance in the Gulf of Eilat were studied at station A1 during 1996–2000. In summer, a deep chlorophyll maximum developed at about 80 m, characterized by high concentrations of Prochlorococcus, while during the mixing time, Prochlorococcus, Synechococcus, and eukaryotic algae were found throughout the water column. Chlorophyll concentrations in the euphotic zone were low (0.1–0.6 μg L⁻¹) and as such, the phytoplankton influence on light attenuation was minor. The vertical attenuation coefficient, K_d (PAR) (photosynthetically available radiation), showed seasonal fluctuation, with a summer minimum of ∼0.04 m⁻¹ and a spring maximum of ∼0.065 m⁻¹. The euphotic zone ranged to depths between 80 and 115 m. Phytoplankton absorption spectra were shown to be dependent on depth. During summer stratification, as a response to the exponential decrease of light in the water column, the phytoplankton exhibited photoacclimation, evident as a marked increase in cellular chlorophyll with increasing depth. Light in the Gulf is not a limiting factor even down to more than 100 m, except when combined with stratification, e.g., nutrient limitation, does it affect phytoplankton abundance and composition.