In Magnaporthe oryzae, pyriform conidia are the primary inoculum and the main source for disease dissemination in the field. In this study, we identified and characterized the COM1 gene that was disrupted in three insertional mutants producing slender conidia. COM1 encodes a putative transcription regulator unique to filamentous ascomycetes. The com1 disruption and deletion mutants had similar defects in conidium morphology and were significantly reduced in virulence on rice and barley seedlings. Microscopic examination revealed that the Deltacom1 mutants were defective in appressorium turgor generation, penetration, and infectious growth. COM1 was expressed constitutively in M. oryzae. The Com1 protein had putative helix-loop-helix structures and three predicted nuclear localization signal sequences. In transformants expressing COM1(335-613)-enhanced green fluorescent protein fusion constructs, fluorescence signals were observed in the nucleus. Our data indicated that the COM1 gene may encode a novel transcription regulator that regulates conidial development and invasive growth in M. oryzae.