  A total dominating set of a graph $G$ is a set $D$ of vertices of $G$ such
that every vertex of $G$ has a neighbor in $D$. A locating-total dominating set
of $G$ is a total dominating set $D$ of $G$ with the additional property that
every two distinct vertices outside $D$ have distinct neighbors in $D$; that
is, for distinct vertices $u$ and $v$ outside $D$, $N(u) \cap D \ne N(v) \cap
D$ where $N(u)$ denotes the open neighborhood of $u$. A graph is twin-free if
every two distinct vertices have distinct open and closed neighborhoods. The
location-total domination number of $G$, denoted $LT(G)$, is the minimum
cardinality of a locating-total dominating set in $G$. It is well-known that
every connected graph of order $n \geq 3$ has a total dominating set of size at
most $\frac{2}{3}n$. We conjecture that if $G$ is a twin-free graph of order
$n$ with no isolated vertex, then $LT(G) \leq \frac{2}{3}n$. We prove the
conjecture for graphs without $4$-cycles as a subgraph. We also prove that if
$G$ is a twin-free graph of order $n$, then $LT(G) \le \frac{3}{4}n$.
