It has been reported that during incubation, large eggs produce more heat than small eggs (Rahn et al., 1974; Hoyt, 1987; Vleck et al., 1980; Vleck and Vleck, 1987; Meijerhof and Van Beek, 1993). Large eggs also face more difficulties to remove the surplus heat from the egg (French, 1997), as a result of the decreasing ratio between egg surface and egg content with increasing egg size (Vogel, 1984), and the reduced air velocity over the eggs in commercial incubators (French, 1997). If large eggs and small eggs are incubated under similar conditions, the higher heat production (HP) and increased difficulties to remove heat in large eggs will result in higher embryo temperatures in these eggs (Meijerhof and Van Beek, 1993, Meijerhof, 2002). The influence of embryo temperature on embryo development and hatchability is shown by Lourens et al (2005), who used egg shell temperature (EST) as reflection of embryo temperature. As egg size influences embryo temperature through heat production and heat transfer, experiments studying the effect of egg sizes on embryo development between species (Ricklefs, 1987) or late embryonic mortality within one species (Hagger et al, 1986; Reinhart and Moran, 1979) will be influenced by differences in embryo temperature, if incubator conditions are not adjusted to obtain an equal embryo temperature. To our knowledge, the effect of egg size on embryo development and hatchability is never studied independent of embryo temperature. Therefore, an experiment was conducted to incubate eggs from two different size classes at an equal EST. The goal of the experiment was to investigate the effect of egg size on HP, embryo development and energy transition between egg and hatchling, when eggs of different sizes were kept on the same EST of 37.8°C throughout incubation.