This study reports an efficient inoculation protocol that allowed cytological analysis of the infection process of the rice false smut pathogen Ustilaginoidea virens. Examination of serial semithin and ultrathin sections of infected spikelets showed that the primary infection sites for the pathogen were the upper parts of the three stamen filaments located between the ovary and the lodicules. The stigma and lodicules were also occasionally infected to a limited extent. The pathogen infected the filaments intercellularly and extended intercellularly along the filament base. The host cells were degraded gradually. The pathogen did not penetrate host cell walls directly and did not form haustoria. In the balls the ovary remained alive and was never infected. This suggests that the pathogen is a biotrophic parasite that grows intercellularly in vivo.