  Following Rivi\`ere's study of conservation laws for second order quasilinear
systems with critical nonlinearty and Lamm/Rivi\`ere's generalization to fourth
order, we consider similar systems of order $2m$. Typical examples are
$m$-polyharmonic maps. Under natural conditions, we find a conservation law for
weak solutions on $2m$-dimensional domains. This implies continuity of weak
solutions.
