alpha-1,2-Mannosidase from Trichoderma reesei was used to modify the N-linked glycosylation pathway of the methylotrophic yeast Pichia pastoris. Expression of foreign influenza glycoproteins with more extensively processed N-linked oligosaccharides was observed when alpha-1,2-mannosidase was secreted in the culture medium. However, intracellular removal of mannose residues may stimulate mannosyltransferase activity and leads to hyperglycosylation. [H-3]Mannose suicide selection or high concentrations of orthovanadate, commonly used to isolate glycosylation mutants of Saccharomyces cerevisiae, had no profound effect on Pichia pastoris.