Abstract: Investigation on a natural epizootic in population of the masson-pine caterpillar,Dendrolimus punctatus,caused by Beauveria bassiana in 2006 spring revealed that the density of the cadavers infected by B.bassiana declined rapidly from 32.6 down to 0.8 capita/m2 in epizootic area(EA)after one year, while those in 3 of 4 non epizootic areas(NEA)rose significantly by 36.8%-178.1%.In May of 2006,inocula in air and soil surface declined rapidly from 6.1 down to 2.3 cfu in EA within one month, while those in NEA fluctuated very slightly and kept relatively stable at low level.In the next year,however, cfu in air and soil in EA and NEA all kept stable at low level,but those in NEA became much higher than those in EA,suggesting that the epizootic became enzootic in EA,while the mycosis got more severe in NEA;the original EA rapidly became NEA,while some NEAs were developing gradually into EAs.The dynamic change was typical of insect epizootics.