| Summary: | <i>Ammannia</i><i>auriculata</i> is a troublesome broadleaf weed, widely distributed in the paddy fields of southern China. In this study, 10 biotypes of <i>A. auriculata</i> were sampled from Yangzhou City, China, where the paddy fields were seriously infested with <i>A. auriculata</i>, and their resistance levels to acetolactate synthase (ALS) inhibitor bensulfuron-methyl were determined. The whole-plant response assays showed that nine <i>A. auriculata</i> biotypes were highly resistant (from 16.4- to 183.1-fold) to bensulfuron-methyl in comparison with a susceptible YZ-S biotype, and only one YZ-6 biotype was susceptible. <i>ALS</i> gene sequencing revealed that three <i>ALS</i> gene copies existed in <i>A. auriculata</i>, and four different amino acid substitutions (Pro197-Leu, -Ala, -Ser, and -His) at site 197 in the <i>AaALS1</i> or <i>2</i> genes were found in eight resistant biotypes. In addition, no amino acid mutations in three <i>ALS</i> genes were found in the YZ-3 biotype. These results suggested that target-site mutations or non-target-site resistance mechanisms were involved in tested resistant <i>A. auriculata</i> biotypes. Finally, a cleaved amplified polymorphic sequence (CAPS) marker was identified to rapidly detect the Pro197 mutations in <i>A. auriculata</i>.
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