Several levan hyperproducing mutants of Zymomonas mobilis strains were selected by mutagenesis with N-methyl-N-nitro-nitrosoguanidine and caffeine. Highest levan production (41 g l(-1)) was obtained with a mutant strain HL 29 in a culture medium containing 200 g sucrose l(-1) and 0.5 g (NH4)(2)SO4 l(-1) stored at 7 degreesC for 29 days. This is the first report describing the levan synthesis by Z. mobilis at 7 degreesC.