A novel slow-growing obligatory heterofermentative nonstarter lactic acid bacterium, Lactobacillus wasatchii sp. nov, was studied for growth and gas production in Cheddar cheese made using a Streptococcus thermophilus starter. Cheesemaking trials were conducted using starter St. thermophilus alone or in combination with Lb. wasatchii deliberately added to cheese milk at a level of ~10⁴ cfu/ml. Then cheeses were ripened at 6 or 12°C. At d 1, starter streptococcal numbers were similar in both cheeses (~10⁹ cfu/g) and nonstarter lactic acid bacteria (NSLAB) counts were below detectable levels (<10² cfu/g). As expected, Lactobacillus wasatchii counts were 3 × 10⁵ cfu/g in cheeses inoculated with this bacterium. Starter streptococci decreased over time at both ripening temperatures but fell more rapidly at 12°C, especially in cheese with Lb. wasatchii (P < 0.05). Populations of NSLAB and Lb. wasatchii reached 5 × 10⁷ and 2 × 10⁸ cfu/g, respectively after 16 wk of ripening at 12°C, and their emergence was correlated with ~0.5% reductions in galactose concentrations. Levels of galactose at 6°C had also similar decrease after 16 wk storage. Gas formation and textural defects were only observed in cheese with added Lb. wasatchii ripened at 12°C. Results demonstrate that, Lb. wasatchii can contribute to late gas blowing in Cheddar cheese made with St. thermophilus, especially when the cheese is ripened at elevated temperature.