  Let Lag(E) be the grassmannian of lagrangian subspaces of a complex
symplectic vector space E. We construct a Maslov class which generates the
second integral cohomology of Lag(E), and we show that its mod 2 reduction is
the characteristic class of a flat gerbe with structure group Z_2. We explain
the relation of this gerbe to the well-known flat Maslov line bundle with
structure group Z_4 over the real lagrangian grassmannian, whose characteristic
class is the mod 4 reduction of the real Maslov class.
