We derive exact Friedmann--Robertson--Walker cosmological solutions ingeneral scalar--tensor gravity theories, including Brans--Dicke gravity, forstiff matter or radiation. These correspond to the long or short wavelengthmodes respectively of massless scalar fields. If present, the long wavelengthmodes of such fields would be expected to dominate the energy density of theuniverse at early times and thus these models provide an insight into theclassical behaviour of these scalar--tensor cosmologies near an initialsingularity, or bounce. The particularly simple exact solutions also provide auseful example of the possible evolution of the Brans--Dicke (or dilaton)field, $\phi$, and the Brans--Dicke parameter, $\omega(\phi)$, at late times inspatially curved as well as flat universes. We also discuss the correspondingsolutions in the conformally related Einstein metric.
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