Undefined mesophilic cheese starters are complex ecosystems that contain both homofermentative and heterofermentative lactic acid bacteria, with the Lactococcus genera representing the former and Lceuonostoc and sometimes Lactobacillus the latter. These starters originate from old butter starters that were selected on their ability to produce aroma compounds in butter. Although the heterofermentative species are present in the starters and in the cheeses made with these starters, they have not been studied to the same extent as the Lactococcus population. Heterofermentative strains isolated from DL-starters including strains from our culture collection were used throughout the project. Initially selected strains were screened for enzyme activities involved in cheese flavour formation after growth in a cheese based medium (CBM) and in a nutrient rich growth medium (MRS). The Leuconostoc strains had low aminopeptidase activity compared to Lactobacillus danicus and especially Le. mesenteroides subsp. cremoris had a low and narrow activity. Aminotransferase activity was high on aromatic amino acids for Lb. danicus, and the Leuconostoc species had an activity similar to Lb. danicus only after growth in CBM. A cheese trial was performed with selected strains to investigate how the heterofermentative strains influenced the ripening in semi-hard cheese. The cheeses were made using a Lactococcus starter including citrate positive Lactoccus and with the addition of one strain of heterofermentative bacteria at the time. Differences were seen in the establishment of the heterofermentative bacteria in the cheese matrix, Le. pseudomesenteroides and Lb. danicus grew to a higher number and survived longer than Le. mesenteroides subsp. cremoris. More secondary alcohols and less acetoin were found in cheeses where the heterofermentative bacteria grew and were established well. The biodiversity of heterofermentative bacteria in DL-starters was examined using pulsed field gel electrophoresis (PFGE) and the strains were characterized with 16s rRNA sequencing and carbohydrate fermentation. The sugar fermentation results indicated an ongoing adaption of Le. mesenteroides to a dairy environment. The adaption from a plant environment to a dairy environment was also indicated in the draft genome sequences of strains Le. pseudomesenteroides 1159, PS12 and Le. mesenteroides subsp. cmreoris T26. The dairy strains had, in comparison with plant isolates, the ability to ferment citrate and lacked several genes involved in the fermentation of complex carbohydrates. The presented research in this thesis has gained insight in to the role of heterofermentative lactic acid bacteria in cheese flavour formation. The traditional DL-starters contained numerous of strains of heterofermentative Le. mesenteroides, whereas Le. pseudomesenteroides and  only were found in some starters. The potential of heterofermentative lactic acid bacteria to effect the flavour formation in cheese seems to be dependent on how well they grow and establish themselves in the cheese matrix. Lb. danicus had metabolic activities that lead to gas formation in the cheese matrix after nine weeks of ripening, indicating that it behaved more like non-starter Lactobacillus in cheese. The genetic information obtained gave insight in the metabolism of Leuconostoc and gave rise to numerous of new topics for further research in this area.