The influence of sex on the meat quality of Lacha (L) and Rasa Aragonesa (RA) lambs was studied. Male and female L lambs were given ewes' milk and were slaughtered at 25 and 24 days of age corresponding to 11·4 and 10·9 kg live weight (LW), respectively. Male and female RA lambs were weaned at 58 days (16·0 kg LW) and then given concentrate and barley straw until slaughter at 89 and 91 days of age, corresponding to 24·5 and 23·1 kg LW, respectively. The parameters measured on the carcasses were: hot carcass weight, cold carcass weight, conformation, colour, firmness and thickness of dorsal fat and colour of muscle rectus abdominis. Parameters measured on longissimus dorsi muscle were: ultimate pH (pHu), instrumental colour (L*, a*, b*), myoglobin concentration, ash, crude protein, fat, moisture and water-holding capacity (WHC). Iodine value and fatty acid composition were studied in the intramuscular (IM), intermuscular (IN) and subcutaneous (SC) fat depots. Females of both breeds had higher amount of IM fat (P < 0·001 and P < 0·05 for L and RA breeds, respectively) and firmer SC fat than males (P < 0·05). There were no differences between sexes in pHu, myoglobin concentration, WHC and the L*, a* and b* colour parameters. No significant differences between sexes were observed in the total saturated fatty acids, total unsaturated fatty acids and in the iodine value in the three fat depots studied in both breeds. However, the females of the breed had significantly higher proportions offatty acids with 15 carbon atoms (pentadecanoic (C15:0)) and with 16 carbon atoms (palmitic (C16:0)and palmitoleic (C16:1)) than the males in the three depots. The females of Rasa Aragonesa breed had a higher proportion of palmitoleic acid (C16:1)in the IM depot (P < 0·001) and of palmitic acid (C16:0)in the SC depot (P < 0·05) than the males. The absence of significant differences in the unsaturated fatty acid content between males and females showed that at the low slaughter weight, sex had no effect on the nature and composition of fat.