Fattening performance and carcass characteristics were evaluated in 202 lambs sired by D’man (D), Timahdite (T) and terminal sires of the Ile-de-France (IF), Lacaune (L), and Mérinos Précoce (MP) breeds. Previous results had suggested that the three latter improved breeds had relatively similar growth and carcass performance in crossbreeding. They were, therefore, pooled in this study into one meat group breed (M) to facilitate comparisons between the five genotypes of interest: two purebreds (T, D), two single crosses (D ✕ T (DT), M ✕ T), and one three-breed cross (M ✕ DT). Results showed important improvements of performance in crossbred lambs especially those sired by meat breed rams. M ✕ T and M ✕ DT lambs grew fastest (238 and 256 g/day, respectively) and showed the best food conversion ratio. D purebred lambs realized the lowest fattening gains (196 g/day) while, T and D ✕ T lambs were intermediate (209 and 218 g/day, respectively). The D lambs deposited relatively more internal fat and relatively less subcutaneous backfat, and their carcasses had the lowest conformation score and smallest longissimus muscle area (11·23 cm2). The carcasses of M ✕ T and M ✕ DT crossbred lambs had larger longissimus muscle areas 13·97 and 13·88 cm2, respectively, thicker subcutaneous fat cover and better conformation scores than other genotypes. The effects of M as a terminal sire breed were favourable for the majority of fattening and carcass characteristics. The absence of differences between M ✕ T and M ✕ DT genotypes on post-weaning growth, food conversion ratio and on objective and subjective measurements of carcass conformation indicated that the infusion of 25% of D genes in M ✕ DT lambs did not affect post-weaning growth and carcass traits. The results on the use of M sires and DT dams, which have above average reproductive rates, in M ✕ DT crossbreeding indicate that a two-stage crossbreeding system could enhance the quantity and the quality of meat sheep production in Morocco.