The optimum temperature range for conidial germination of Pyriculacia oryzae on a slide glass was $26{sim}30^{circ}C$, at which at least four hours of leaf wetness period was required to germinate. Conidial germination was significantly reduced under dry conditions (relative humidity<85%) at $34^{circ}C$ but not at lower temperature (18, 22, 26, $30^{circ}C$). Number of lesions developed were greater at $26^{circ}C$ than at other temperature tested. The average leaf wetness period required for production of a lesion per plant was 22 hours at $18^{circ}C$, 16 hours at $22^{circ}C$, 10 hours at $26^{circ}C$, and 8 hours at $30^{circ}C$. Less than one lesion per plant occurred at $34^{circ}C$ even under 24 hours of leaf wetness period. The time period between inoculation and lesion appearance was $7{sim}8$ days at $18^{circ}C$, $4{sim}5$ days at $22^{circ}C$ and $26^{circ}C$, and $3{sim}4$ days at $30^{circ}C$. The time period required for lesion appearance after inoculation was not affected by leaf wetness period and relative humidity. Lesion length increased most rapidly at $30^{circ}C$ during the first four days after lesion appearance. Thereater, the rate of increase in lesion length was geratest at $26^{circ}C$. The average increment of lesion length per day when relative humidity was greater than 90% was 0.7mm at $18^{circ}C;and;22^{circ}C$, 1mm at $26^{circ}C$, and 0.8mm at $30^{circ}C$. When relative humidity was less than 85%, the increments of lesion length per day were approximately $50{sim}60%$ of those under humid conditions (relative humidity>90%) at all temperature regimes except $30^{circ}C$. Relative humidity did not significantly affected lesion length at $30^{circ}C$.