The behaviour of Streptococcus thermophilus in combination with Lactococcus lactis subsp. cremoris or subsp. lactis mesophilic starters in experimental Cheddar cheese is reported. In a standard manufacturing procedure employing a 38 °C cook temperature, even very low levels (0·007%) of Str. thermophilus combined with normal levels of the mesophilic starter (1·7%) resulted in increased rates of acid production, the formation of significant amounts of galactose (∼ 13 mmol/kg cheese), and populations nearly equivalent to those of the mesophilic lactic starter in the curd before salting. At a 41 °C cook temperature, the Str. thermophilus attained a higher maximum population (∼ log 8·2 colony forming units (cfu)/g) than the Lc. lactis subsp. cremoris (∼ log 6·8 cfu/g) and formed more galactose (∼ 28 mmol/kg). Lactobacillus rhamnosus, deliberately added to a cheese made using Str. thermophilus starter and which contained 24 mmol galactose/kg at day one, utilized all the galactose during the first 3 months of cheese ripening. Adventitious non-starter lactic acid bacteria had the potential to utilize this substrate too, and a close relationship was demonstrated between the increase in this flora and the disappearance of the galactose. Some possible consequences for cheese quality of using Str. thermophilus as a starter component are discussed.