Distinguishing among eggs of large gull species in mixed colonies is difficult because egg size is variable, size ranges overlap and colors are similar. Regional and yearly differences in egg size of Herring Gulls (Larus argentatus) were compared among three regions (Bay of Fundy, Newfoundland, and low Arctic). In two of these regions (Newfoundland and Bay of Fundy), eggs of Herring Gulls and Great Black-backed Gulls (L. marinus) were measured and discriminant analysis models were created to distinguish between the eggs of these two species. Egg dimensions of Herring Gulls decreased from low Arctic (largest) to Bay of Fundy to Newfoundland (smallest). In both species, where a = first-laid egg, b = second-laid, and c = third-laid, a- and b-eggs were of similar size, but c-eggs were significantly smaller; measurements of a- and b-eggs were pooled. The only annual differences were in a- and b-eggs (treated separately) in Newfoundland; there were no annual differences in c-eggs or in a/b-eggs combined. There were regional differences in a/b-eggs combined, but not in c-eggs. Three separate discriminant function models were created for Newfoundland a/b-eggs, Bay of Fundy a/b-eggs, and Newfoundland/Bay of Fundy c-eggs. Models discriminated 90% or more of the eggs. Length and diameter differ between species and must both be measured to discriminate between Herring and Great Black-backed gull eggs; diameter alone is not reliable. Future application of such models will improve identification of clutches in field situations and lead to more accurate gull population estimates.