Heterotrophic nanoflagellates and naked ciliates have been isolated from open sea waters and separately grown by using free living marine bacteria as food. The modal diameters of purified (by differential screening) populations are about 3 and 12.5 μm, respectively. The spectral determination of their absorption and scattering properties have been carried out. By combining these values with the cell number density and size distribution function, simultaneously measured, the efficiency factors for absorption and scattering at the level of individual cells can be derived. From these factors and by using the anomalous diffraction theory, the relative index of refraction can be derived (at least for flagellates); it agrees with the predictions made from the intracellular carbon concentrations (mean value 129 kgC m–3). In addition, backscattering efficiency (not determined) is computed through Mie theory. Like bacteria, these larger protists exhibit the typical absorption maximum of cytochrome (at about 410–415 nm). The impact of these organisms upon the properties of oceanic waters is compared to that of the bacterial compartment, in terms of scattering and backscattering coefficients. For that, the bacterial number-to-chlorophyll concentration relationship, as proposed by Cole et al. (1988), is adopted and reasonable assumptions resting on field data are made concerning the respective biomasses of bacterioplankton and heterotrophic nanoplankton. An assessment of the comparative role of the phototrophic and heterotrophic communities is also attempted. Algal cells in open ocean, and to a lesser extent small heterotrophs, dominate the scattering coefficients; the sum of their contributions is close to the coefficient measured in various (from oligotrophic to eutrophic) waters. On the contrary, these organisms are definitely insignificant contributors to the backscattering coefficient. In the backscattering process, heterotrophic bacteria are the more efficient agents; they, however, account only for a small part of the coefficient as actually derived from measurements at sea. The main contributors would presumably be the abundant population of very tiny, poorly known particles with size similar to, or below that of commonly identified bacteria. This partly speculative conclusion, based on theoretical considerations, is supported by recent studies demonstrating the importance of submicrometer detrital particles.