Disclosed herein are novel Pichia pastoris strains for expression of exogenous proteins with substantially homogeneous N-glycans. The strains are genetically engineered to include a mutant OCH1 allele which is transcribed into an mRNA coding for a mutant OCH1 gene product (i.e., α-1,6-mannosyltransferase, or “OCH1 protein”). The mutant OCH1 protein contains a catalytic domain substantially identical to that of the wild type OCH1 protein, but lacks an N-terminal sequence necessary to target the OCH1 protein to the Golgi apparatus. The strains disclosed herein are robust, stable, and transformable, and the mutant OCH1 allele and the ability to produce substantially homogeneous N-glycans are maintained for generations after rounds of freezing and thawing and after subsequent transformations.