Several hypotheses predict that nest defense should increase as the nesting cycle progresses, but the predicted pattern of increase differs. Previous studies of nest defense in gulls and terns have given conflicting results. Most of these have treated colonies as a unit, thus breeding asynchrony may have obscured temporal patterns. Common Tern (Sterna hirundo) nest defense against Great Black-backed Gulls (Larus marinus) was observed during two years, and responses of individuals or pairs of known nesting stage were recorded. Responses of terns to observers were also recorded. Mobbing of gulls did not increase as incubation progressed, contradicting parental investment models, but did increase as chicks aged. Chicks were defended more strongly by parents than were eggs. Responses to Great Black-backed Gulls were more frequent during the second wave of nesting, when the potential for re-nesting would be very low. There was no association between clutch size and nest defense. Aggression by terns to observers was rare during incubation, but became common immediately after hatching and remained frequent until the chicks fledged. The temporal pattern of mobbing in the semi-precocial Common Tern most closely resembles that predicted for species with altricial young. However, interpretation is complicated by the fact that gulls prey more frequently on chicks than eggs.