In France, a variable proportion of intensively reared lamb carcasses have a lack of firmness in the subcutaneous adipose tissues which could lead to commercial depreciation. Several factors could be responsible for this phenomenon. In this study, a beet pulp based diet and a cereal rich diet were tested on 48 'Lacaune' breed male lambs whose paternal origin was known. The incorporation of beet pulp increased the proportion of fatty acids (FA) with low melting point (odd-numbered or branched chain FA) in subcutaneous adipose tissues. It decreased the proportion of unsaturated even-numbered FA in all adipose tissues. The paternal origin seemed to have some effects on several FA but at a lower level than diet. The FA composition of adipose tissue was dependant on sampling site. Dorsal adipose tissue seemed to be the most sensitive, followed by caudal and internal adipose tissues.