The nereid polychaete Laeonereis culveri (Webster) reproduces in the atokal condition in the Mystic River Estuary, Connecticut. Females spawn demersal eggs, 135-162 µ in diameter, that do not extrude a jelly layer upon fertilization and that give rise to unciliated embryos. Embryogenesis leads directly to a 3-setigen larva in four days (at 22 ± 3° C; 30‰). The trochophore is suppressed due to the retarded development of locomotory cilia which do not appear until the 3-setiger larva has formed. Although provided with ciliary tracts and capable of swimming, L. culveri larvae are normally benthic in habit. They reside in burrows within the upper 2 cm of fine, flocculent sediments where they feed chiefly on benthic diatoms until developing past the 5-setiger stage after which they become non-selective deposit feeders. The primary function of the larval ciliature is to pass water currents through the burrow. In the laboratory, 3-to 5-setiger larvae alternately swim and crawl in the absence of sediments on in the presence of coarse sediments (particle diameters > 250 µ); but they readily burrow into and remain within fine sediments (particle diameters < 250 µ) even if the latter are relatively free of organic matter. Swim-crawl behavior also appears to be elicited under conditions of unfavorable water quality. When the sixth setiger forms, the larva is no longer capable of swimming by means of its cilia. Development beyond the 3-setger stage approximates that of other species except that L. culveri larvae are apparently unique in ultimately developing eight nototrochs (one per each of the first eight trunk segments). The nototrochs are retained and function in burrow ventilation until the 16-to 18-setiger stages when body undulations commence creating ventilation currents. Larval development is completed with addition of the eighth trunk segment. At this stage, the tentacular segment (first trunk segment) is incorporated into the peristomium and the first pair of parapodia are modified to form the posterodorsal tentacular cirri. Sigmoid growth curves resulted when either length or segment-formation was used as an indicator of growth among laboratory-reared worms. Sexually mature, laboratory-reared worms were only about one-half the length of sexually mature worms in a natural population. Thirty-two individuals (17 females, 15 males) of an F1 generation were reared to maturity and spawned after 168-198 days of development. F2 generation larvae were identical in morphology to those of the F1 generation.