Larval behavior and metamorphosis in Parasmittina nitida morphotype B from the Gulf of Mexico has been studied. The larvae have two basic types of movement: (1) a clockwise-counterclockwise movement about the aboral-oral axis of the lobular larval form resulting in either slow horizontal or rapid vertical movement, and (2) a directed horizontal movement of the creeping larval form, whereby either the oral lobe is pressed against the substrate or the aboral-oral axis is tilted forward. In both forms, the vibratile plume of the pyriform organ complex extends the leading edge of the larva. Metamorphosis was observed with Nomarski differential interference microscopy in living specimens and with scanning electron microscopy in fixed specimens. Polypide development— in particular, the formation and diminution of the nutritive mass, the differentiation of the polypide rudiment, diaphragm, vestibular glands, operculum, major components of the musculature and alimentary canal, and the early stages of astogenetic growth—is described. The tata ancestrula of this species is characterized by a frontal wall calcified distally to the aperture, which is surrounded by nine erect spines. The polypide feeds actively within seven to eight days after the onset of larval attachment and metamorphosis under laboratory conditions of 22°C.