Motile intensity of the cells, defined as the specific mean kinetic energy, was measured by image analysis and used to formulate a suitable medium for the cultivation of a motile microalga. Nitrogen source at 60 mg/L was used as the target component. The cells grown in a medium containing urea showed the highest motile intensities during cultivation when compared to the cells grown with NH4Cl or (NH4)(2)SO4 as the nitrogen source. The urea culture gave the highest biomass yield and lipid content of 1.06 +/- 0.12 g/L and 22.34 +/- 2.56 %, respectively after 150 h of cultivation. These results indicated a strong dependence of motile intensity on the nitrogen source used in the growth medium. This concept, which requires only a small droplet of the sample, can find potential application in the design and optimization of culture media. (c) 2006 Elsevier Ltd. All rights reserved.