With the objective of evaluating the impact of genotype by environment interaction (G by E) on breeding values for 150-day weight in Mexican Katahdin sheep, data from a total of 41,323 lambs, the progeny of 1,862 sires, were used to estimate genetic correlations between seven regional environments representing the majority of Mexico. Estimates of heritability within environments ranged from 0.24 ± 0.04 for Pacific Central to 0.42 ± 0.11 for North East. Genetic correlations across environments averaged 0.51 across all pairs, ranging from 0.07 ± 0.61 to 0.86 ± 0.27, indicating the presence of G by E interaction. A validation study predicted progeny performance within each environment with and without sire by flock effects from sire breeding values (EBVs) calculated from single trait BLUP analyses of data in the remaining environments. The regression of offspring performance on sire EBV were predictable across en vironments, but at lower levels than the expected value, in the absence of G by E, of 0.5. Fitting sire by flock improved the predictability with the regression coefficient increasing from 0.31 to 0.36.