  We give necessary and sufficient conditions for a few classes of known
circulant graphs and/or digraphs to be singular. The above graph classes are
generalized to $(r,s,t)$-digraphs for non-negative integers $r,s$ and $t$, and
the digraph $C_n^{i,j,k,l}$, with certain restrictions. We also obtain a
necessary and sufficient condition for the digraphs $C_n^{i,j,k,l}$ to be
singular. Some necessary conditions are given under which the
$(r,s,t)$-digraphs are singular.
