  We study upper bounds on the size of optimum locating-total dominating sets
in graphs. A set $S$ of vertices of a graph $G$ is a locating-total dominating
set if every vertex of $G$ has a neighbor in $S$, and if any two vertices
outside $S$ have distinct neighborhoods within $S$. The smallest size of such a
set is denoted by $\gamma^L_t(G)$. It has been conjectured that
$\gamma^L_t(G)\leq\frac{2n}{3}$ holds for every twin-free graph $G$ of order
$n$ without isolated vertices. We prove that the conjecture holds for
cobipartite graphs, split graphs, block graphs and subcubic graphs.
