Larval compound (jointed) setae of the polychaete Nereis vexillosa Grube were examined by scanning and transmission electron microscopy and by polarization microscopy. Long-bladed spinigers and short-bladed falcigers are described. The proximal shaft of each of these types of setae flares distally into a serrated collar and encloses the proximal end of a toothed blade. The collar projects on one side as a boss. The blade and the cortex of the shaft have longitudinal channels. A large excentric cavity in the shaft (the medullary channel) contains a loose meshwork of trabeculae. In the distal part of the shaft these trabeculae are aggregated into diaphragms. The seta is invested with an electron dense layer of enamel. Juvenile setae contain both chitin and protein. With respect to the long axis of the seta, the blade and the cortex of the shaft are positively birefringent and the medullary diaphragms are negatively birefringent. KOH extraction renders the setae negative to a test for protein and reverses the sign of birefringence of the cortical material of the shaft.