Weaning weight records available for the South African Mutton Merino breed wereutilised to investigate the possibility of a genotype by environment interaction in the breed. A direct heritability estimate of 0.308 ± 0.022 was obtained. Including flock-year-season x sire interaction (FYSXS) led to a direct heritability estimate of 0.017 ± 0.00 and a c2FYSXS timate of 0.111 ± 0.00. Although flocks are genetically related by the use of rams from other flocks, very few of these rams are used in different flocks in the same year. Four flocks were hence selected where this did not seem a problem. This led to a direct heritability estimate of 0.125 ± 0.061 when interaction was included and a c2 FYSXS estimate of 0.107± 0.019. The results indicate that a genotype x environment interaction in South African Mutton Merino exists. It is, however, impossible to quantify the effect of the interaction in a breed analysis because of the nature of the data structure. This is a general problem in sheep in South Africa, partly owing to the lack of use of Al across flocks.