Candida albicans, Candida tropicalis, Candida grabrata, and Cryptococcus neoformans are causative pathogens of opportunistic diseases in immunocompromised human patients. Silkworms are killed by injection of these pathogenic fungi into their hemolymph. In this paper, we describe recent results by our laboratory and other researchers using gene-deficient strains of these pathogenic fungi. The silkworm is considered to be a useful model animal for understanding the pathogenicity of these fungi. Silkworms are also beneficial for evaluating therapeutically active anti-fungal reagents.