  Sh. Mozes showed that the commensurator of the lattice ${\rm PSL}_2
\bigl({\bf F}_p[t{}^{-1}] \bigr)$ is dense in the full automorphism group of
the Bruhat-Tits tree of valency $p+1$, the latter group being much bigger than
${\rm PSL}_2 \bigl({\bf F}_p((t)) \bigr)$. By G.A. Margulis' criterion, this
density is a generalized arithmeticity result. We show that the density of the
commensurator holds for many tree-lattices among those called of Nagao type by
H. Bass and A. Lubotzky. The result covers many lattices obtained via Moufang
twin trees.
