Abstract: In order to study the relationship between the number of reproductive generation of Alternaria alternata in field and its sensitivity to fungicide, the sensitivity of six isolates of Alternaria alternata from different tobacco growth stages to fungicides were determined indoors, and the sensitivity differences between the isolates were analy zed with cluster analysis method.The results showed that the isolate's sensitivity to Dimethachlon was significantly different, the EC₅₀ of isolates No.4, 1, 2 and 3 was 193.3644, 102.3738, 42.0390 and 58.4109mg/L, respectively.No.4 had the least sensitivity.From these data, no obvious relationship was found between the number of isolate's generation and its sensitivity to Dimethachlon.There were both sensitive and insensitive isolates in different tobacco growth stages.The difference of sensitivities to fungicides could be viewed from the dendrogram of cluster analysis also.