O-glycosylation has been considered a limiting factor in protein secretion in filamentous fungi. Overexpression of the yeast DPM1 gene encoding dolichylphosphate mannose synthase (DPMS) in an Aspergillus nidulans mutant (BWB26A) deficient in O-glycosylation caused an increase in the number of secretory vesicles and changes in protein secretion. However, the secretory proteins, primarily O-mannosylated glucoamylase and N-glycosylated invertase, were mainly trapped in the periplasmic space. Different glycoforms of invertase were found insite the cells, in the periplasmic space and in the cultivation medium. Our data point to the importance of the cell wall as a barrier in protein secretion.Elucidation of the regulatory mechanisms and limiting factors in O-glycosylation of secreted proteins is an important task, since the efficiency of protein secretion appears to be related to their O-glycosylation (Kubicek et al., 1987;Kruszewska et al., 1999;Agaphonov et al., 2001). It has been shown for Trichoderma reesei that inhibition of O-glycosylation, in Vol. 52 No. 1/2005 195-205 QUARTERLY .