Adhesin, hydrolytic enzyme, and germ tube formation are regarded as major virulent factors of Candida albicans. In the pathogenesis of Candida albicans, the first step is adherence to host tissue with adhesin, and then invasion to the tissue by secreting hydrolytic enzymes like protease and phospholipase. The fuzzy coat is of interest as it may play important roles in antigenicity, phagocytosis, and adherence factors that are important in the virulence of C. albicans. The virulent factors could be expressed higher in vivo than in vitro and serum is similar to in vivo condition. Therefore, using transmitting electron microscopy, we studied the effects of serum on fibril expression of C. albicans YWMC132. The fibrillar layer of C. albicans was expressed only in serum The fibril expression was not inhibited at concentrations of C. albicans until $8{ imes}10^7;cells/ml$. However, the thickness of the fibril layer was reduced from $100{sim}130;nm$ to 20 nm according to increases in cell concentrations from $2{ imes}10^7;cells/ml$ to $8{ imes}10^7;cells/ml$. The fibrillar layer of C. albicans was expressed in serum, early, within 10 min, but the germ tube formation was expressed only after 60 min. When the fibrillar C. albicans was treated with lyticase, protease, or dithiothreitol (DTT), the fibrillar layer disappeared. The protein concentration was higher in the fibrillar cell than in the nonfibrillar cell. These results indicated that the proteinous fibrillar layer of C. albicans YWMC132 was expressed only in serum culture.