Acid-catalyzed hot-water treatment for efficient extraction of lipids from a wet microalga, Chlorella vulgaris, was investigated. For an initial fatty acids content of 381.6 mg/g cell, the extracted-lipid yield with no heating and no catalyst was 83.2 mg/g cell. Under a 1% H2SO4 concentration heated at 120 degrees C for 60 min, however, the lipid-extraction yield was 337.4 mg/g cell. The fatty acids content, meanwhile, was 935 mg fatty acid/g lipid. According to the severity index formula, 337.5 mg/g cell of yield under the 1% H2SO4 concentration heated at 150 degrees C for 8 min, and 334.2 mg/g cell of yield under the 0.5% H2SO4 concentration heated at 150 degrees C for 16 min, were obtained. The lipids extracted by acid-catalyzed hotwater treatment were converted to biodiesel. The biodiesel's fatty acid methyl ester (FAME) content after esterification of the microalgal lipids was increased to 79.2% by the addition of excess methanol and sulfuric acid. (C) 2013 Elsevier Ltd. All rights reserved.