A strain of Penicillium daleae has been isolated from a forest soil sample for its ability to degrade monomeric rhamnogalacturonan-II (mRG-II), a complex pectic polysaccharide ubiquitous in the primary plant cell wall. Monomeric RG-II, a most unusual polysaccharide in terms of composition and structure, is resistant to all known pectinolytic enzymes used in the fruit- and vegetable-processing industry. P. daleae has been cultured in a minimum mineral medium supplemented with 0.5% mRG-II as the sole carbon source. The degradation of the substrate in the culture supernatant was followed by high performance size-exclusion chromatography. P. daleae growth was supported by the degradation of 75% of the initial mRG-II. Monomeric RG-II degradation was followed by gas chromatography-mass spectrometry of the alditol acetates and trimethylsilylated derivatives of the constitutive monosaccharides at different times in the culturing process. Sequential mRG-II degradation led to a resistant core after 31 days of culture, representing 25% of the initial molecule which has been characterized. The degradation of mRG-II indicated that P. daleae is a potential source of new pectinases whose mode of action requires further elucidation Such enzymes seemed to be exposed to the surface of fungal cell walls or were present in the periplasmic compartment.