Interactions between Lactobacillus brevis subsp. lindneri CB1, L. plantarum DC400, Saccharomyces cerevisiae 141 and S. exiguus M14 from sourdoughs were studied in a co-culture model system using a synthetic medium. The lack of competition for maltose when S.exiguus M14 was present in co-culture with each of the lactic acid bacteria (LAB) enhanced the bacterial cell yield and lactic and acetic acid production. L.brevis subsp. lindneri CB1 resting cells hydrolysed maltose and accumulated glucose in the medium, allowing the growth of maltose negative yeasts. S.cerevisiae 141 competed greatly with each of the LAB for glucose and only with L.plantarum DC400 for fructose, causing a decrease in the bacterial cell number and in acid production. As a result of the glucose and fructose availability after the invertase activity of both yeasts, L.plantarum DC400 grew optimally in the presence of sucrose as a carbon source. All of the interactions indicated were confirmed by studying the behaviour of the co-cultures in wheat flour hydrolysate.