Aim: To develop a diagnostic assay based on polymerase chain reaction for the detection of Magnaporthe grisea from infested rice seeds. Methods and Results: Primers were designed based on the nucleotide sequence of the mif 23, an infection-specific gene of M. grisea. The primers amplified target DNA from genetically and geographically diverse isolates of the pathogen. The lowest concentration of template DNA that led to amplification was 20 rho g. No PCR product was detected when DNA from other fungi was used, indicating the specificity of the primers. With this PCR based seed assay, M. grisea was detected in rice seedlots with infestation rates as low as 0.2%. Conclusion: The PCR detection of M. grisea is simple, rapid, specific, sensitive and suitable for the routine detection of the pathogen in infested seeds. Significance and impact of the Study: Introduction of the blast fungus into new areas where it has not been previously recorded could be avoided by the detection of infested seedlots. A PCR-based seed assay could facilitate risk assessment of naturally infested rice seeds; help design management programs and optimize fungicide use.