SUMMARY The rice blast fungus Magnaporthe grisea causes one of the most destructive diseases of rice. To initiate the infection of host tissues, conidia elaborate germ tubes that differentiate specialized infection structures called appressoria. Microarrays composed of 3500 cDNAs of M. grisea were prepared for the identification of genes that are specifically up- or down-regulated during appressorium formation. Gene expression in ungerminated conidia, during appressorium formation, and during mycelial growth was investigated with a novel highly sensitive dendrimer based detection system. Transcripts of 85 different genes were found to be more abundant in ungerminated conidia and/or in conidia with developing appressoria than in vegetative mycelia. Nineteen of these showed higher expression in both ungerminated conidia and developing appressoria than in mycelia, suggesting that their expression remains elevated during the early stage of fungal infection. The expression of 18 genes was higher in ungerminated conidia than in developing appressoria, indicating their possible role in the germination process or maintaining dormancy. Transcripts of 47 genes were found to be more abundant in developing appressoria than in ungerminated conidia, suggesting that their expression is induced during appressorium formation. Several of these genes, including a chitin binding protein and infection structure specific protein MIF23, were previously shown to be preferentially expressed during appressorium formation. However, the expression of many of these genes has not been reported prior to this analysis. In contrast, transcripts of 38 different genes were found to be more abundant in mycelia than in developing appressoria. A Northern blot analysis of selected genes was consistent with the microarray results. Results from this study provide a powerful resource for furthering our understanding of gene expression during infection-related morphogenesis and for the functional analysis of M. grisea genes involved in fungal infection.