An investigation was made in order to ascertain the mode of variation of length and breadth of eggs of the black-headed gull (Larus r. ridibundus) from a colony on the northeastern side of the island of Texel in the Netherlands. The colony can be divided into two ecologically different areas. A difference could be shown in the measurements of eggs collected in these areas. Two possible explanations of this coincidence are given. Methods of egg collecting and measuring are discussed, and data and tables of the findings during 1950, 1951 and 1952 are furnished. On account of the flood and disaster of 1953 the investigation was broken off before its completion. The fact that mean measurements and weights must be used with due caution is emphasized.