Details of mouth formation in normal and exogastrulated Pisaster ochraceus larvae have been studied by light microscopy and transmission and scanning electron microscopy. As the archenteron begins to bend, the cells in the presumptive mouth region dissociate and migrate into the blastocoele where they become mesenchyme cells. This leaves a defect in the “blind” endodermal tube, which is covered by a basal lamina. Subsequently this exposed basal lamina bulges to form a blister which appears to extend across the blastocoele to make contact with spikelike projections from the future stomodeal region of the ectoderm. Mesenchyme cell processes are associated with both the basal lamina blister and the ectoderm in this region and may provide both motive power and guidance for contact. Shortly after contact is made the blister of basal lamina from the endoderm fuses with the basal lamina of the ectodermal cells and the ectoderm begins to invaginate. At this time the lateral walls of the presumptive oesophagus are largely formed of naked basal lamina with some loosely associated cells on the endodermal side. Eventually the lateral walls of the proximal part of the oesophagus become cellular, giving rise to an epithelium. A cell plug located between the stomodeum and oesophagus persists for some time before finally breaking down to complete the larval digestive tract. Experiments with exogastrulae suggest that many of these developmental patterns are determined before gastrulation.