Terns of the genus Chlidonias may be exceptional among the larids in laying the ultimate egg in the clutch-sequence that is not smaller than the earlier eggs. To investigate this, we analyzed data on egg length, breadth, volume and shape in three-egg clutches of the White-winged Tern (Chlidonias leucopterus) breeding in NE Poland. Both length and volume of the c-egg was larger than mean values the a- and b-eggs in the clutch. They also differed in shape. Egg breadth did not differ in such a comparison. Two species of Chlidonias terns do not conform to the typical pattern of larids, in which the last-laid egg is the smallest in the clutch; this may be related to their being insectivorous rather than piscivorous. Chlidonias terns lay eggs of the size expected from the regression of mean egg size on mean female body size in all the Western Palearctic terns.