Nuclei were isolated from mycelium of the filamentous fungi Fusarium moniliforme and Trichoderma reesei, and fused with protoplasts of the xylose-fermenting yeast Pichia stipitis using PEG as the fusogenic agent. Hybrids which were morphologically yeastlike and able to hydrolyze xylan showed a similar chromosomal pattern to the parental yeast when clamped homogeneous electric field (CHEF) electrophoresis was done. These hybrids were isolated and their xylanase activity determined quantitatively. Xylanase activity in the parental fungal species was maximum at 96 h growth; it reached 796 nkat ml(-1) in the culture medium for F. moniliforme and 898 nkat ml(-1) at 120 h growth for T. reesei. Xylanase activity in one of the hybrids reached 350 nkat ml(-1) in 9 h. P. stipitis itself showed no xylanase activity.