BACKGROUND: Microalgae have recently been considered a potential feedstock for biodiesel production, since they do not compete with agricultural land, unlike oil crops. However, the production processes must be energetically and economically viable. Therefore, an in situ methanolysis process is proposed for biodiesel production directly from microalgae biomass, to avoid the need for the separation and extraction steps. RESULTS: Biodiesel was obtained using methanol as the methylation reactant for the transesterification reaction and hydrochloric acid as the catalyst precursor, at 80 degrees C for 2 h of reaction. A mass return of 23.07 +/- 2.76% (m/m) was obtained. Spectrometry in the infrared region showed that the product had equivalent bands of axial deformation of C=O, C-O and C-H, i.e. an ester. Tests showed the chromatographic profile of fatty acids in the sample. A process energetic efficiency value of 1.17 was obtained for microalgae derived biodiesel, which is higher than from soybean and sunflower, reportedly 1.06 and 1.12. CONCLUSIONS: Industrial sustainability results from low energetic, economic and environmental losses. The microalgae in situ methanolysis process showed greater fuel available energy than energy consumption, therefore is energetically sustainable. Economic and environmental issues should still be addressed. (C) 2011 Society of Chemical Industry