Nassarius obsoletus is primarily a deposit-feeder. The microflora to be found on the surface of sediment of intertidal flats serves as its major source of food. Living bivalves do not form part of the diet as previously reported. Dead organisms such as molluscs, crustacea, and fish are eaten when available but are not a principal item of food. Occasionally thallus algae are probably also utilized. Ecologically,Nassarius obsoletus may be regarded largely as a herbivorous species and deposit-feeder. In the strict sense, however, it is an omnivore and a facultative scavenger. Growth ofNassarius obsoletus occurs principally during the summer months. The winter is passed in a state of quiescence below mean low water. Zero- and one-year classes are readily distinguished but after the third summer the various age groups can no longer be identified by the length-frequency method. The longevity ofNassarius obsoletus cannot be directly determined but some individuals probably live to at least five years. A growth of 1.3 to 1.4 mm per month was estimated during the first two summers in a New England population found in Great Pond, Falmouth, Massachusetts. An increase in length of 3 to 5 times that at the time of metamorphosis occurs during the first summer’s growth.