Well diffracting crystals of rhamnogalacturonan acetylesterase from Aspergillus aculeatus have been obtained in two polymorphic modifications despite its heterogeneous glyco­sylation. The best-diffracting crystals (resolution 1.55 Å) are orthorhombic. The limit of the diffraction pattern of the other (trigonal) form is 2.5 Å. The ability of the enzyme to crystallize appears to depend on the glycosylation of the protein sample. This aspect has been investigated by mass spectrometry, which also showed that the orthorhombic crystals have the same glycosylation as the protein sample used in the crystallization.