  This paper shows that there are symplectic four-manifolds M with the
following property: a single isotopy class of smooth embedded two-spheres in M
contains infinitely many Lagrangian submanifolds, no two of which are isotopic
as Lagrangian submanifolds. The examples are constructed using a special class
of symplectic automorphisms ("generalized Dehn twists"). The proof uses Floer
homology.
  Revised version: one footnote removed, one reference added
