  The strong-field limit for the 2-form potential on an M5-brane yields a
conformal chiral 2-form electrodynamics in six dimensions, with gauge-invariant
self-interactions but no adjustable coupling constant; the stress tensor is
that of a null fluid. Lorentz invariance can be made manifest via an
interpretation as a tensionless `space-filling M5-brane', or as a truncation of
the infra-red dynamics of an M5-brane in AdS7 x S4.
