To evaluate the bacterial reverse mutation of xylooligosaccharide(XO)s the in vitro Ames test using Salmonella typhimurium (TA98, TA100, TA1535, TA1537) and Escherichia coli (WP2 uvrA) was performed. XO was negative in Ames test with Salmonella typhimurium and Escherichia coli with and without rat liver microsomal enzyme (S-9 fraction). According to the results, XO does not cause bacterial reverse mutation.