Atlantic white-sided dolphins (Lagenorhynchus acutus) are among the most abundant, and least studied, cetaceans in coastal New England. Between April and October 1984 through 1997 we sighted 1,231 groups of Atlantic white-sided dolphins, primarily on Stellwagen Bank and Jeffreys Ledge (two shallow glacial deposits along the coasts of Massachusetts, New Hampshire, and Maine). Mean group size was 52 (≥90.9), and was significantly larger from August through October (71.9 ± 111.4) than April through June (35.0 ± 45.4). Calf sightings were uncommon until June and July, after which they were present in over 50% of groups. Combined with observations of apparent newborn calves, this confirms that early summer is an important calving period. The presence of calves did not, however, solely account for the increase in group size. Boat interaction (bow- and stern-wake riding) was the most commonly recorded behavior (47.4% of sightings), followed by traveling (31.4%), interactions with other cetacean species (27.6%), social interaction (15.5%), and feeding (9.5%). While feeding was uncommon, one observation of apparently coordinated “ball” feeding was seen with sand lance (Ammodytes spp.) as the visible prey. Aerial behavior showed a positive correlation with group size, although it was often impossible to tell whether the same dolphins were leaping repeatedly. Eighty-eight dolphins were photo-identified using either unusual body pigment or a distinctive dorsal fin. While several individuals were reidentified between years and between areas, no reidentifications were made within a year in the same area. Unusually pigmented individuals were much more likely to be reidentified than those with distinctive dorsal fins, most likely due to higher visibility. We suggest that Atlantic white-sided dolphins are generally using the study area as transients in what appears to be a large home range.