A screening procedure has been set up for isolating microorganisms capable of liquefying coal. Spanish coals were used in the tests, namely a brown lignite from Galicia, sub-bituminous coal from Teruel and hard coal from Minas Figaredo S.A. (Asturias). Among the isolated strains several microorganisms proved capable of liquefying untreated lignite. When lignites were pretreated a more intense and rapid liquefaction was achieved, chelating agents being among the best pretreatments. None of the isolated microorganisms could satisfactorily liquefy sub-bituminous and hard coals in solid media. On the other hand, some fungi grew specifically on the untreated coals, engulfing them and in many cases a soft slurry was obtained. Several of the isolated microorganisms were able to solubilize all three untreated Spanish coals in liquid media. Coal solubilization was measured spectrophotometrically at 300, 400 and 450 nm.