Introduction The improvement of sheep productivity requires efficient actions on different components, mainly on prolificacy. The use of prolific breeds in the creation of synthetic breeds allows this objective to be attained and the wishes of breeders who like to have one breed type which permits replacement from their own flock to be realized. Nevertheless, the efficiency of such a breeding system is related to the heterosis at the first generation and to the proportion of retained heterosis in the subsequent generations of inter se mating ( Young et al. 1986 ). A programme for the creation of a synthetic breed of sheep having 50% D’man and 50% Sardi was initiated ( Boujenane & Bradford 1991), and preliminary results were reported ( Boujenane & Chafik 1994). The objectives of this study were to analyse the reproduction, growth and survival performance of Sardi, D’man and F1 to F5 generations of crossbred sheep, and to calculate the heterosis and the heterosis retained at different generations. Material and methods Animals The study was undertaken at the Tadla Farm of the Institut Agronomique et Vétérinaire Hassan II. The farm is located at 32.5°N, 7°W at approximately 415 m elevation in an irrigated area of the interior of Morocco, 20 km north of the High Atlas mountain, 150 km east‐northeast of Marrakech and approximately 160 km from Casablanca on the Atlantic Coast. The analysis has concerned 1418 litter records of 751 ewes and 1702 growth records of 1996 lambs born from 97 sires. The mean age and weight at mating of the ewes were 34 months and 42 kg, respectively. The average number of lambings was 2.4. Data were collected during six lambing periods from 1990 to 1995 inclusive. The matings were made to establish a new synthetic breed. Ewes were of Sardi, D’man, F1, F2, F3 and F4 breed groups, whereas lambs were of Sardi, D’man, F1, F2, F3, F4 and F5 breed groups. In this study the F1 is defined as the first generation that reflects the final breed composition of the synthetic breed which was established by using the same sires and dams used in the parental breeds. Animals of the F1 to F5 generations were produced by inter se mating. Crossbred ewes were mated to rams of their own breed group, but purebred D’man (D) and Sardi (S) ewes were mated to both D’man and Sardi rams in order to produce purebred and reciprocal F1 (S × D and D × S) lambs. Details concerning the parental breeds and the origin of their samples were reported by Lahlou‐Kassi et al. (1989) . Management Ewes were managed under an annual lambing system. They were mated for the first time at 16 months of age. Mating periods started on average at June 25 and lasted 40 days. Each year, ewes were placed at random in different pens, with an average of 20–25 ewes per ram. Mating was performed indoors at night. The rams were used for 1 or 2 years. Apart from the mating period, all ewes and their subsequent lambs were subjected to the same management. The ewes were kept on pasture (fallow and wheat stubble), except during the ...