ABSTRACT.1Pegomya nigritarsisZetterstedt is a gregarious leaf‐mining fly attackingRumexsp. The larvae from one clutch of eggs all inhabit the same mine.2Egg‐batch size varies between host plants and is correlated with leaf size.3OnRumex obtusifoliusL. larval survival as a function of clutch size was measured in the field. This measure was incorporated in an optimal clutch size model and a clutch size prediction obtained.4The predicted optimal clutch size, three, is that most frequently found in the fi