We consider locally equi-continuous strongly continuous semigroups on locally convex spaces (X, τ ) that are also equipped with a 'suitable' auxiliary norm. We introduce the set N of τ -continuous semi-norms that are bounded by the norm. If (X, τ ) has the property that N is closed under countable convex combinations, then a number of Banach space results can be generalised in a straightforward way. Importantly, we extend the Hille-Yosida theorem. We relate our results to those on bicontinuous semigroups and show that they can be applied to semigroups on (C b (E), β) and (B(H), β) for a Polish space E and a Hilbert space H and where β is their respective strict topology.