The biodiversity of lactic acid bacteria(LAB) involved in production of koumiss, a traditional fermented mare milk product in Xinjiang of China, were studied. A total of 152 LAB strains were isolated from 30 home-made koumiss samples and identified using standard conventional identification methods. The predominant strains proved to be Lactobacillus(L.) helveticus(51.3% of the total isolates), L.acidophilus(18.4%) and L.casei subsp.pseudoplantarum(8.6%), while strains of L.gasseri, L.casei subsp.casei, L.curvatus,L.sanfrancisco,L.coryniformis subsp.coryniformis,L.brevis, L.ruminis, L.plantrum,L.homohiechill,L.fermentum,L.dellbrueckii subsp.bulgaricu, L.crispatus,L.farciminis and L.hilgardii were also present in lower numbers(1～4strains). 8 strains of the isolates can't be identified according to present methods. The results obtained showed L.helveticus and L.acidophilus were present in all koumiss samples and L.helveticus was the predominant strain.