From 1990-1997, we observed food deliveries to Common (Sterna hirundo) and Arctic (S. paradisaea) tern chicks in the Gulf of Maine; deliveries were observed at four Common Tern colonies from 1991-1997 and at three Arctic Tern colonies from 1990-1997. Dietary overlap was high, 0.812. Fish comprised over 96% of the prey delivered (frequency) to Common Tern chicks. In contrast, Arctic Tern chicks were fed a diet consisting of 65% fish and 35% invertebrates. Although the two terns fed different proportions of fish, both species relied heavily on white hake (Urophycis tenuis), Four-bearded rockling (Enchelyopus cimbrius) and Atlantic herring (Clupea harengus). Amer- ican sand lance (Ammodytes americanus) was an important prey for Common Terns at one island. Overall, Common Terns fed their young a greater diversity of prey than did Arctic Terns. We found significant inter-annual and inter- colony differences in the diversity and proportions of prey types. "Boom and bust" years were evident for particular prey. We suspect dietary differences in Maine were due to colony location; however, a detailed study of the relation- ship between prey availability and abundance and the foraging ecology of terns in Maine is needed to elucidate these relationships. We believe long-term diet data sets gathered from multiple sites are the best way to characterize waterbird diets.