Plant debris floating on the water surface after puddling was collected and applied in a sclerotia-free experimental field to confirm that mycelia in plant debris act as primary inocula for rice sheath blight in the Philippines. As we were able to isolate sheath blight fungus from plant debris, the mycelium apparently survived in debris until the following growing season. The percentage of diseased hills in plots without plant debris reached 3.9% by one month after heading, whereas in plots containing plant debris at a rate of 2kg and 4kg per 35m², the values were 11% and 18%, respectively. Inoculum potential of sclerotia was about three times as much as that of plant debris. These results suggest that mycelium in plant debris may play a role as primary inoculum of the disease in the Philippines.