Prototroch formation was studied in the polychaete Nereis virens using light, scanning electron, and confocal laser microscopy. Cell lineage of trochoblasts was followed and chronology of their appearance was determined. The prototroch ciliary ring is formed by twelve descendants of micromere 1m ². The remaining four primary trochoblasts have no cilia and, together with descendants of accessory trochoblasts, become anterior supporting cells of the prototroch. Posterior supporting cells are formed by secondary trochoblasts, which are derived from the second micromere quartet 2m. The results obtained make it possible to analyze one of the ancient programs of animal development.