  Using Brownian dynamics computer simulations we show that a two-dimensional
suspension of self-propelled ("active") colloidal particles crystallizes at
sufficiently high densities. Compared to the equilibrium freezing of passive
particles the freezing density is both significantly shifted and depends on the
structural or dynamical criterion employed. In non-equilibrium the transition
is accompanied by pronounced structural heterogeneities. This leads to a
transition region between liquid and solid in which the suspension is globally
ordered but unordered liquid-like "bubbles" still persist.
