In this note we extend the spectral theorem for bimodules
to the higher rank graph $C^*$-algebra context. Under the
assumption that the graph is row finite and has no sources, we
show that a bimodule over a natural abelian subalgebra is
determined by its spectrum iff it is generated by the
Cuntz-Krieger partial isometries which it contains iff the
bimodule is invariant under the gauge automorphisms. We also
show that the natural abelian subalgebra is a masa iff the
higher rank graph satisfies an aperiodicity condition.