The objective of this research was to describe the effect of dietary protein content on the uniformity of egg production in ISA-Brown and Hy-Line laying strains. Six dietary protein levels (120–220 g protein/kg feed) were each fed to 16 individually caged hens, per treatment and strain, during the first 6 weeks of the trial from 28 weeks of age. During the second phase, from 35 weeks, only one feed was offered, this containing 175 g protein/kg. Egg production, feed intake, egg weight, egg output and changes in bodyweight were measured. Some birds were sampled before the trial began, after 6- and again after 10-weeks for carcass analysis. Maximum egg output differed between strains but the marginal response to dietary protein was the same in both strains, the coefficients of response being 220 mg protein/g egg output and 9.0 g per kg bodyweight. The coefficient of variation in egg output was low in both strains fed the highest protein feed but increased as the dietary protein level dropped, with the biggest increase occurring in outputs between birds fed 140 and 120 g protein/kg. These increases were particularly marked in the ISA strain, being almost twice as high as those of the Hy-Line strain. Similarly the lowest coefficients of variation in daily food intake were on the highest protein feeds, with a 2- to 3-fold increase on the lowest dietary protein levels, but with both strains in this case showing similar degrees of uniformity. Variation in body lipid content was higher in the ISA strain between dietary treatments. Uniformity in egg output is increased at the highest intakes of dietary protein because the amino acid requirements of an increasing proportion of the population are met by these higher protein contents. As the protein supply becomes marginal and then deficient uniformity is decreased not only because the most demanding individuals cannot consume sufficient to achieve their potential, but also because birds differ in their ability to deposit excess energy as body lipid when attempting to consume sufficient of a feed limiting in protein. This ability to fatten differs not only between individuals within a population but between strains, as shown in the differences between the two strains used in this trial.