AbstractThe variation in Skeletonema cells grown at 3 klux continuous illumination and 20°C is reported. Four different types of lamps gave no difference in the photosynthetic characteristics. The average diameter of the cells decreased from 8–3.5 μ during their six months vegetative period. The ratio between the pigment content in the largest and the smallest cells was about 2:1. A good correlation between cell volume and chlorophyllacontent was found for this species. The content of chlorophyllcgenerally varied between 4 and 17 per cent of the chlorophyllacontent. — A distinct correlation between the chlorophyllacontent and the rate of photosynthesis per unit of cells at low light intensity was found. The rate of photosynthesis, in mg C per mg chlorophyllaand hour at 1 klux, varied between 0.40 and 0.70 for all 60 experiments with an average value of 0.56. The corresponding value for cells deficient in phosophorus was 0.19 and for cells deficient in nitrogen 0.09. — The material also showed a good correlation between the rate of photosynthesis per cell at 1 klux and the light‐saturated rate of photosynthesis. Ikvaried between 7 an