The putative vesicle transport protein Vac1p of the human pathogenic yeast Candida albicans plays an important role in virulence. To determine the cellular functions of Vac1p, a null mutant was generated by sequential disruption of both alleles. The vac1 null mutant strain showed defective endosomal vesicle transport, demonstrating a role of Vac1p in protein transport to the vacuole. Vac1p also contributes to resistance to metal ions, as the null mutant strain was hypersensitive to Cu(2+), Zn(2+) and Ni(2+). In addition, the loss of Vac1p affected several virulence factors of C. albicans. In particular, the vac1 null mutant strain showed defective hyphal growth, even when hyphal formation was induced via different pathways. Furthermore, Vac1p affects chlamydospore formation, adherence to human vaginal epithelial cells, and the secretion of aspartyl proteinases (Saps). Avirulence in a mouse model of systemic infection of the vac1 null mutant strongly suggests that Vac1p of C. albicans is essential for pathogenicity. In summary, the Vac1p protein is required for several cellular pathways, in particular those that control virulence and pathogenicity. Consequently, Vac1p is a novel and interesting target for antifungal drugs.