  A total dominating set of a graph G with no isolated vertices is a subset S
of the vertex set such that every vertex of G is adjacent to a vertex in S. The
total domination number of G is the minimum cardinality of a total dominating
set of G. In this paper, we study the total domination number of middle graphs.
Indeed, we obtain tight bounds for this number in terms of the order of the
graph G. We also compute the total domination number of the middle graph of
some known families of graphs explicitly. Moreover, some Nordhaus-Gaddum-like
relations are presented for the total domination number of middle graphs.
