A novel nonstarter lactic acid bacterium, Lactobacillus wasatchii sp. nov. has been studied for growth and gas formation in a control Cheddar cheese and in cheese supplemented with 0.5% ribose, 0.5% galactose, or 0.25% ribose plus 0.25% galactose using regular and accelerated cheese ripening temperatures of 6 and 12°C. Cheese milk along with starter lactococci was inoculated with Lb. wasatchii at a level of 10⁴ cfu/mL, whereas a control vat was inoculated with starter lactococci only. Starter numbers in both cheeses decreased from 10⁷ to ~10³ cfu/g at 23 wk of ripening at both temperatures, except the control cheese at 6°C which had one log higher final cell counts. Unlike starter bacteria, nonstarter lactic acid bacteria started at <10² cfu/g in the cheese and reached 10⁶ to 10⁷ cfu/g with higher numbers observed at 12°C. Lactobacillus wasatchii grew to ≥10⁸ cfu/g in cheese supplemented with ribose (alone or with galactose) at elevated temperature which was ~1-log higher compared with the control and galactose-supplemented cheeses. In all cheeses with adjunct Lb. wasatchii, highest growth and gas formation was observed at 12°C although most gas production occurred at ≥16 wk. Adding both ribose and galactose provided substantially higher growth and gas formation because of the ability of Lb. wasatchii to co-utilize both sugars; producing gas from galactose as a result of the obligatory heterofermentative nature of the bacterium. Even without sugar supplementation, gas was observed in the presence of adjunct Lb. wasatchii after 16 wk. We have observed that Lb. wasatchii can grow to high cell densities when grown in carbohydrate-restricted broth containing lactococcal cell lysate. During cheese ripening, lysis of starter bacteria would provide sufficient substrate (such as ribose) to allow growth of Lb. wasatchii during cheese ripening and the presence of any hexoses in cheese would allow Lb. wasatchii to produce gas. We conclude that Lb. wasatchii is a previously undetected contributor to late gas formation in Cheddar cheese and the defect is more pronounced when elevated ripening temperatures are used.