1. The data on the fine structure of the chloroplasts and the chlorophyll analyses, in two algal flagellates, Euglena gracilis and Poteriochromonas stipitata, are consistent with the assumption that the chlorophyll molecules are arranged in monomolecular layers at the interfaces between the lipid (dense) and the aqueous protein complex layers. 2. The cross-sectional area available to each chlorophyll molecule in the chloroplasts of Euglena and Poteriochromonas was found to be 222 x 10(-16) cm(2). and 246 x 10(-16) cm(2). By comparison, the cross-sectional area of the hydrophilic head of the chlorophyll molecule is approximately 240 x 10(-16) cm(2). 3. The model of the molecular network depicted in Text-fig. 1 is purely speculative. It predicts a lower limit for the number ratio of chlorophyll to other pigment molecules of about 1:1 and a weight ratio of 2:1. A loose packed structure such as that shown in Text-fig. 1 predicts a weight ratio of 4:1 to 6:1. These values bracket the large majority of experimental values found for this ratio. The relative constancy of the number of chlorophyll molecules per chloroplast and the volume of the chloroplast indicated by the data in Table III suggest that chloroplasts probably possess a similar structural arrangement in a variety of photosynthetic microorganisms and plants.