A study was conducted to compare carcass traits in live animals in a group of lambs of three different genotypes (Merino precoz, Suffolk and crossbreds Suffolk x Merino precoz). Two ultrasonographic measurements were made with an interval of 15 days in the lumbar zone between the 12th and 13th rib, determining depth (PL, cm), width (AL, cm) and area (AOL, cm(2)) of the Longissimus dorsi muscle and backfat thickness (EGD, cm) at that level. Variables were analyzed using analysis of variance, considering lamb's genotype, date of assessment and the effect of age and weight as sources of variation, which were entered into the model as covariates. Additionally, simple correlations were determined between them. Values for AOL, PL, AL and EGD at constant age (140 days) and constant weight (38.9 kg) were not affected by lamb's genotype (P > 0.05), suggesting that the characteristics of their carcasses would be similar. Nevertheless, Merino precoz lambs tended to have a greater EGD in the first measurement. PL was highly and positively correlated with AL and AOL, as well as with EGD so that this measurement can be used to estimate muscle development. However, PL should be considered in conjunction with EGD in the indices that evaluate in vivo lamb carcass quality in order to prevent excessive adiposity of this.