To know the primary infectious sources for the infection to the insects, we investigated the survival, germination and multiplication of Matarhizium anisopliae in the soil of mulberry plantation. The fungus, M. anisopliae, was detected in number of 102-103/g soil throughout the year by the dilution plate method using CRCC agar from the soil of ordinary mulberry plantation. However, seasonal changes in number of the fungus were not found in the field test. The fungus isolates detected from the soil of the field were pathogenic to silkworm larvae almost as of high pathogenecity as the isolate from the diseased silkworm. In the soil infested respectively with conidia of three isolates, each isolate of M. anisopliae was detected in number of 102-104/g soil for 25 months in the field test. Seasonal fluctuation of density of the fungus was not recognized in the soil examined. The conidia of M. anisopliae germinated easily in untreated soil from the fields after 20 hours at 25°C, and new conidia were produced in chain at the tip of single for small phialide after 48 hours at 25°C. The conidia taken from conidiophores of M. anisopliae survived only 10-30 days at 25°C in laboratory test. Longevity of this fungus conidia was shorter than other entomogenous fungi, Beauveria bassiana, B. tenella, and Aspergillus flavus. The results obtained clearly suggest that the fungus M. anisopliae is distributed widely in the soil of mulberry plantation, and the fungus can grow in the soil.