  We determine the spectrum of the sub-Laplacian on pseudo H-type nilmanifolds
and present pairs of isospectral but non-diffeomorphic nilmanifolds with
respect to the sub-Laplacian. We observe that these pairs are also isospectral
with respect to the Laplacian. More generally, our method allows us to
construct an arbitrary number of isospectral but mutually non-diffeomorphic
nilmanifolds. Finally, we present two nilmanifolds of different dimensions such
that the short time heat trace expansions of the corresponding sub-Laplace
operators coincide up to a term which vanishes to infinite order as time tends
to zero.
