Three lines of Tribolium castaneum were raised totally on a yeast-free or yeast-supplemented diet, or raised to pupation on a yeast-free diet and then on a yeast-supplemented diet, and vice versa, to study the effects of genotype x environment interaction and diet changes after pupation on reproduction.Feeding the yeast-supplemented diet generally resulted in earlier sexual maturation, heavier adult weights and higher egg production. The diets had no effects on larval viability. Changes in diet after pupation had no effects on age at sexual maturity or adult weight. Egg production was determined largely by the environment just prior to and during egg laying. However, egg production on the yeast-free diet was influenced by the pre-pupation diet.There was significant line x environment interaction effects on age at sexual maturity, mature egg production and adult weight (P<0.01). There was also a sire x environment effect on egg production (P<0.05).