Hydrophobins are small proteins found in filamentous fungi and characterized by their ability to change the character of a surface by spontaneous self-assembly on a hydrophobic-hydrophilic interface. Hydrophobin HFBII from Trichoderma reesei was crystallized by the hanging-drop vapour-diffusion method at 293 K. Two crystal forms were obtained: a native form and a form crystallized in the presence of manganese chloride. The native crystals were of high symmetry, cubic I23, but only diffracted to 3.25 A. The crystals grown in the presence of manganese were monoclinic and diffracted to 1.0 A with a synchrotron-radiation source. The anomalous difference Patterson map calculated from the home laboratory data showed a strong single peak, possibly caused by manganese present in the crystallization solution.