The protein import capacity of peroxisomes in methylotrophic yeasts was studied using Pichia pastoris containing one or two extra copies of the gene encoding the peroxisomal protein alcohol oxidase. The alcohol oxidase overproduced in this strain was only partially imported and assembled into the active, octameric form of the protein. The excess remained in the cytosol as protein aggregates composed of monomers. These results are discussed in view of the possible application of peroxisomes as storage compartments for heterologous proteins.