A commercial culture of Kefir grew well at 30 degreesC and 37 degreesC with NaCl up to 4% (w/v) but at 42 degreesC and 4% (w/v) NaCl it grew very slowly. These conditions were selected for the mutation with N-methyl-N-nitrosoguanidine in order to improve cell yield at high temperatures and salt concentration. The mutated cells had higher biomass and growth rate compared with the original culture. The improvement of Kefir at high temperatures and salt concentrations offers advantages for uses in single cell protein production and alcohol production from whey containing high salt.