A flat inclined modular photobioreactor (FIMP) for mass cultivation of photoautotrophic microorganisms is described. It consists of flat glass reactors connected in cascade facing the sun with the proper tilt angles to assure maximal exposure to direct beam radiation. The optimal cell density in reference to the length of the reactor light path was evaluated, and the effect of the tilt angle on utilization of both direct beam as well as diffuse sunlight was quantitatively assessed. The mixing mode and extent were also optimized in reference to productivity of biomass. The FIMP proved very successful in supporting continuous cultures of the tested species of photoautotrophs, addressing the major criteria involved in design optimization of photobioreactors: Made of fully transparent glass, inclined toward the sun and endowed with a high surface-to-volume ratio, it combines an optimal light path with a vigorous agitation system. The maximal exposure to the culture to solar irradiance as well as the substantial control of temperature facilitate, under these conditions, a particularly high, extremely light-limited optimal cell density. The integrated effects of these growth conditions resulted in record volumetric and areal output rates of Monodus subterraneus, Anabana siamensis, and Spirulina platensis.