Let G be a graph with no isolated vertex. A semitotal forcing set of G is a (zero) forcing set S such that every vertex in S is within distance 2 of another vertex of S. The semitotal forcing number
$F_{t2}(G)$
is the minimum cardinality of a semitotal forcing set in G. In this paper, we prove that it is NP-complete to determine the semitotal forcing number of a graph. We also prove that if
$G\neq K_n$
is a connected graph of order
$n\geq 4$
with maximum degree
$\Delta \geq 2$
, then
$F_{t2}(G)\leq (\Delta-1)n/\Delta$
, with equality if and only if either
$G=C_{4}$
or
$G=P_{4}$
or
$G=K_{\Delta ,\Delta }$
.