  We prove some basic properties of Donaldson's flow of surfaces in a
hyperkahler 4-manifold. When the initial submanifold is symplectic with respect
to one K\"ahler form and Lagrangian with respect to another, we show that
certain kinds of singularities cannot form, and we prove a convergence result
under a condition related to one considered by M.-T. Wang for the mean
curvature flow.
