We report the development of a simple model for assessing the ability of the fungal pathogen Candida albicans to invade the chorioallantoic membrane (CAM) of fertilized hens' eggs. Wild-type and mutant strains of C. albicans were inoculated onto CAM surfaces either as a liquid suspension or on a sterile filter disc. Invasion of the membrane led to death of the embryo due to damage of the CAM, which could be examined histologically to show cell distribution and morphology, and by RT-PCR for assessment of patterns of fungal gene expression in vivo. Prophylactic or co-administration of fluconazole with the inoculum protected the embryo from infection. Secretory aspartyl protease (Sap) mutant strains with reported attenuation of virulence were virulent in the CAM model. However, a C. albicans strain with mutations in two transcription factors Efg1 and Cph1 was unable to form hyphae on the CAM or to penetrate it. The chick CAM, therefore, represents an experimentally tractable and inexpensive alternative to rodent or tissue culture-based invasion models, and can be used to investigate fungal pathogenesis and the genetic regulation of infection and membrane penetration of C. albicans.