The petrology of the Fishguard Volcanic Group and associated rocks is described from a remapped area of Cambrian and Ordovician rocks between Fishguard and Newport, N Pembrokeshire (Dyfed). The volcanic rocks are mainly rhyolitic in composition and consist of pyroclastic flows, falls, lavas, epiclastic deposits, and alloclastic intrusions; welded pyroclastic flows comprise the major component. A small area of basaltic pillow lavas and hyaloclastites is associated with the rhyolitic rocks. The volcanic rocks are accompanied by numerous sheets of dolerite and gabbro, together with diorites and microgranitic intrusions. The sequence at Lower Fishguard Harbour shows an intimate association of welded and non-welded pyroclastic flows with graptolitic shales and other marine sediments, which suggests that welding may have occurred subaqueously. The cooling rate of submarine pyroclastic flows and lavas is thought to be significantly reduced by the formation of a steam jacket and a quenched marginal crust, both providing low thermal conductivity. The hydrostatic pressure of seawater maintains cohesion and retains heat and volatiles within the flow. In addition, water saturation of the rhyolitic melt throughout its cooling maintains mobility and is conducive to subsequent welding.