The comparative cortical growth of the tibiotarsus was studied in 2 turkey strains, one light, one heavy, between 3 and 12 weeks of age, using histological and histomorphometrical methods. The growth in thickness started earlier and was much faster and larger in the heavier turkey strain. The early ovoid shape of the cortex of the heavier strain and the difference between the thickness of the narrow and the thick cortical faces seemed to be related to the action of strong mechanical strains. Nevertheless the diaphyseal cortical ratio remained not significantly different in the 2 strains from the age of 3 weeks. The light strain exhibited during all the studied periods indicated a more variable mineral apposition rate and earlier and more extensive bone remodelling phenomena. Most of the growth in thickness parameters were significantly higher in the heavier strain when turkeys of the 2 strains were compared at an equivalent weight.