The action of levan sucrase from aerobacter levanicum on sucrose can be divided into three reactions: levan formation, inversion and oligosaccharide formation. As in dextran synthesis, levan has extremely high molecular weights which are formed even at very small conversions. The oligosaccharides consist mainly of 1β-fructosyl sucrose. From kinetic data of reactions with enzyme preparations which have been prepared on sucrose containing cultivation media, we could conclude that a levanase was present in our enzyme preparations. By application of fructose instead of sucrose in our cultivation media we could get enzyme preparations completely free of levanase and levan. Temperature- and pH-stability measurements and kinetic data lead to the conclusion that all three reactions are catalysed by the same enzyme. This was confirmed by investigations in the ultracentrifuge; the molecular weight of the enzyme as measured by sedimentation analysis was found to be 22 000.