In field surveys of sugarbeet in Hokkaido, larvae of a Sciarid fly, Pnyxia scabiel (Hopkins) (Sciaridae), were often found feeding on sclerotia and hyphae of Rhizoctonia solani Kuhn AG-2-2, the causal agent of sugarbeet root rot. Population of the larvae increased rapidly with the root rot severity and occasionally up to 5,000 larvae per diseased root were counted. The larvae were found mostly on the lesions or in the soil within a distance of 2 cm from the root surface. When sclerotia were buried in the soil around the diseased or healthy roots of sugarbeet for 3 weeks in autumn in small plastic containers (10 or 250 μm mesh) sclerotial destruction occurred only in the 250 μm containers around the diseased roots, and larvae of P. scabiei were often seen inside these containers. Tests using sterilized or non-sterilized soil with sclerotia of R. solani revealed that when larvae were placed in a paper-pot, damping-off disease of sugarbeet could be prevented. However the disease was not suppressed by the dead larvae killed by isoxathion (insecticide) or propylene oxide. These results suggest that the insect reduces the sclerotial density of R. solani in soil leading to the decrease of the incidence of Rhizoctonia root rot of sugarbeet.