Free-living coccoid bacteria have been repeatedly isolated from the Mediterranean Sea and then cultured in detritus- and predator-free seawater, either unenriched or slightly nutrient-supplemented. The modal diameter of these cultured cells was in the 0.5 to 1 μm range. The spectral values (within the 400–750 nm λ-range) of absorption, total scattering and backscattering have been determined, together with cell enumeration, size distribution and C–N content. Because of their smallness, bacteria are amenable to simplified theoretical expressions providing their optical efficiency factors. Experimental data are in remarkable quantitative agreement with theoretical predictions. As expected, scattering is wavelength (λ−2) dependent and varies according to the fourth power of the size, whereas backscattering is neutral and depends upon the square of the size. Albeit weak, absorption has been detected and distinctly exhibits a maximum around λ = 413 nm. As deduced from the optical properties, the refractive index (about 1.05 with respect to that of water) agrees with the cell material composition, namely with the intracellular carbon concentration (mean value 228 kgC m&minus3).The chemical composition and optical properties of bacterial cells, when combined with the cell number concentrations as related to the chlorophyll concentration (Cole et al., 1988), allow the role of bacterioplankton to be estimated in oceanic waters of various trophic situations. The bacterial compartment appears to form a significant contribution to the light scattering within the upper part of the ocean as well as in forming a sizeable fraction of the particulate organic carbon pool.