  The equations of motion for a self-interacting self-dual tensor in six
dimensions are extracted from the equations describing the M-theory five-brane.
These equations are presented in a self-contained, six-dimensional
Lorentz-covariant form. In particular, it is shown that the field-strength
tensor satisfies a non-linear generalised self-duality constraint. The
self-duality equation is rewritten in five-dimensional notation and shown to be
identical to the corresponding equation in the non-covariant formalism.
