  We study the canonical structure of the Bergshoeff-Hohm-Townsend massive
gravity, linearized around a maximally symmetric background. At the critical
point in the space of parameters, defined by $\Lambda_0/m^2=-1$, we discover an
extra gauge symmetry, which reflects the existence of the partially massless
mode. The number of the Lagrangian degrees of freedom is found to be 1. We show
that the canonical structure of the theory at the critical point is unstable
under linearization.
