Formins are conserved eukaryotic proteins playing key roles in regulating cell polarity. We have characterized the roles of a formin CaBni1p in the polymorphic fungus Candida albicans. CaBni1p localized persistently to hyphal tips during hyphal growth but to distinct growth sites at different cell cycle stages during yeast growth. Cabni1Delta yeast cells exhibited several morphological defects, such as round and enlarged cells, widened bud necks and a random budding pattern. Although Cabni1Delta cells could still undergo yeast-hypha growth switch, the hyphae were markedly swollen. Cabni1Delta also showed defects in spindle and cytoplasmic microtubule orientation and positioning. Coincidentally, the spindle orientation protein CaKar9p in Cabni1Delta yeast cells appeared as multiple random cortical spots, in contrast to the single spot at the bud tip of many wild-type cells. Interestingly, several defects manifested in Cabni1Delta yeast cells were partially corrected during hyphal growth. We found that the second formin CaBnr1p was recruited to hyphal tips, while it localized only to the bud neck during yeast growth. This behavior of CaBnr1p may play a key role in correcting Cabni1Delta defects during hyphal growth. Cabni1Delta exhibited reduced virulence in mice. These results indicate that the formins play an important role in Candida albicans polarized growth and CaBni1p's function is required for virulence.