I've read "On Sense and Reference" and Foundations of Arithmetic. In the latter, which was published earlier, he says that he finds the view that a concept simply is its extension (= it's referent?) to be unobjectionable, although he tries to avoid resting his argument on it. On the other hand "On Sense and Reference" seems to suggest that the referent is distinct from the concept, which seems to represent a change of view. I'm hideously unqualified as a philosopher of language though, so I'm not sure I am willing to assert that without someone agreeing with me first. So do people who've read more Frege/philosophy of language agree?