Aims: To characterize the non-spoilage variant obtained from beer-spoilage Lactobacillus brevis ABBC45C and to identify a potential genetic marker capable of discriminating beer-spoilage L brevis strains from non-spoilers. Methods and Results: A non-spoilage variant was obtained from beer-spoilage L. brevis ABBC45C by repeatedly subculturing the strain at 37degreesC. Genetic characterization of the variant revealed that 12 605 bp portion of one plasmid, designated pRH45II, was lost in the variant. The sequence analysis indicates the presence of 12 ORFs in the deleted region of pRH45II. The PCR and Southern hybridization study revealed that the homologues of ORF5 found in the deleted region were present in all of the beer-spoilage L. brevis strains examined in this study. In contrast, the homlogues appeared to be absent in non-spoilage L. brevis strains. Conclusions: The presence or absence of ORF5 homologues was found to be highly correlated with the beer-spoilage ability of L. brevis strains, indicating this ORF is potentially a useful genetic marker capable of differentiating beer-spoilage strains among L. brevis. Significance and Impact of the Study: A non-spoilage variant was successfully isolated from beer-spoilage L. brevis ABBC45(C). This study could facilitate the understanding of mechanisms underlying beer-spoilage ability of L. brevis.