Seedling trap method was examined to estimate suitable mixed planting ratio of Sasanishiki isogenic lines for effective control of rice blast disease from 1996 and 1997. Mixed seedlings of rice cv. Sasanishiki and its isogenic lines were exposed for six to seven days in the paddy field, and number of infected seedlings with rice blast fungus, Pyricularia grisea, was counted after the exposure. Number of the infected seedlings in the mixtures was correlated with ratio of susceptible isogenic lines or cultivar in them to the predominant pathogenic race of the blast fungus in the field. The result suggests that seedling trap is useful to estimate the suitable mixing ratio of the isogenic lines for effective control of the blast disease.