ABSTRACT: To study relationships between environmental conditions and reproductive output, numbers and sizes of eggs produced by the intertidal bivalve Macoma balthica were determined after the winters of 1995 and 1996 at 3 stations at different intertidallevels in the Dutch Wadden Sea. At all field stations significantly more (1.5 to 7 times) eggs were produced after the cold winter of 1996 than after the mild winter of 1995 when individual body masses were lower than in early 1996. At 2 stations the eggswere also significantly larger in 1996. In both years, eggs were larger at low than at high mud flats. Egg size was significantly positively correlated with adult body mass in the preceding summer (when gametogenesis takes place). Egg numbers, on theother hand, were only significantly positively correlated with the body mass just prior to spawning. Below a body mass of 5.6 mg ash-free dry mass per cm³ (cubic shell length), M. balthica did not produce any eggs. Above this body mass,egg numbers increased by about 7700 per mg ash-free dry mass at a shell length of 15 mm.