Thermophilic lactic acid bacteria, propionibacteria (PAB) and facultatively heterofermentative lactobacilli (FHL) form the main flora of Swiss-type cheeses. The aim of this work was to investigate their interactions and impact on product quality, and in particular on the defect of late fermentation. For this purpose Emmental model cheeses were produced according to a twice-replicated 24 full factorial experimental design. The four factors were: (1) The type of cultures of PAB, one with a weak (Prop 96) and the other one with a strong aspartase activity (Prop 90). (2) The addition or not of a culture of FHL composed of 3 Lactobacillus casei strains. (3) The addition or not of a culture of 4 Lactobacillus helveticus strains. (4) The season: winter (hay feeding) or summer (grass feeding). FHL and PAB counts, organic acids, proteolysis, eye formation were followed during ripening. Prop 90 showed a higher growth and fermentation role resulting in a higher number of eyes and an increased risk of late fermentation compared to Prop 96. PAB growth was also favoured in cheeses manufactured with winter milk which had a slightly higher water content. The addition of L. helveticus tended to increase the risk of late fermentation. On the other hand the addition of FHL inhibited lactate fermentation, with a more marked effect on Prop 96 culture. In conclusion the defect of late fermentation can be prevented by using PAB with weak aspartase activity, by adding FHL and by omitting L. helveticus