Some integrability conditions of the distributions involved on a semi-slant submanifold of a trans-Sasakian manifold are obtained and the geometry of the leaves of these distributions is studied. It is also shown that every totally umbilical semi-slant submanifold \(M\) of a trans-Sasakian manifold \(\overline M\) is a totally geodesic submanifold in \(\overline M\).