Medium screening and optimization is one of the most important preconditions for photoautotrophic cultivation of microalgae. Although, it has been widely conducted indoors, little work performed outdoors. There are enormous differences between indoor and outdoor conditions, especially for light intensity, temperature and their diurnal or annual fluctuations, which would greatly influence microalgae growth. No data shows whether the differences would lead to different results on medium screening and optimization. In present study, medium screening for the photoautotrophic cultivation of Chlorella pyrenoidosa was carried out indoors and outdoors firstly, and then the selected medium was optimized. The results showed that F-Si medium is the optimum both under indoor and outdoor conditions. Based on F-Si medium, nutrients were optimized as follows: NaNO3 500 mg l (1), NaH2PO4 center dot 2H(2)O 7.7 mg l (1) and FeCl3 center dot 6H(2)O 6.30 mg l (1). With the optimized medium, the biomass, lipid content and productivity were all significantly higher both indoors and outdoors. (C) 2014 Elsevier Ltd. All rights reserved.