Laboratory bioassays were conducted to assess the pathogenicity of 24 wild-type isolates of Metarhizium anisopliae toward Nilaparvata lugens. LT50 values ranged from 5 days (highly pathogenic) to > 14 days (nonpathogenic). In vitro germination, growth rate, spore production, extracellular enzyme production, and spore characteristics were assessed in relation to pathogenicity. Rapid germination and growth, and small spore volume, were correlated with high pathogenicity. Highly pathogenic isolates produced no detectable extracellular amylase and detectable amounts of extracellular chitinase, lipase, and protease activity. Evidence for the stable diploid nature of var. majus isolates is presented.