In order to evaluate the use of terminal crossing, liveweight and growth of lambs from birth to weaning were compared in Suffolk (S), Merino (MP) and crossbred Suffolk x Merino (SMP), grazing on Chilean semiarid Mediterranean grasslands. The trial began with mating and finished with weaning, four months after lambing started. Variables analyzed were birth weight, adjusted liveweight at 60, 90 and 120 days of age, and liveweight gain between these periods. Prior to statistical analysis, data were corrected by birth type and maternal age. Sources of variation were lambs 'genotype and sex. Effects were important but significant changes were observed in the order of merit of the genotypes evaluated at different periods until weaning. At 90 days, liveweight of S and SMP was similar and only S was significantly higher than MP (5.8%). However, weight of SMP lambs was 5.4 and 11.6% higher than S and MP at 120 days, respectively (P <= 0.05). This occurred because the daily liveweight gain between 90 and 120 days was 51.0 and 115.5% higher than S and MP, respectively. Even though males weighed more at birth (3.9%) and at 90 days (7.1%), this difference was not significant at 120 days (2.8%; P = 0.1637). These results suggest that the terminal crossing used is justifiable only if the objective is to obtain lambs weighing 35 or more kg at weaning.