There is increasing interest in the development of later lambing systems for crossbred ewes which more closely match ewe nutritional requirements with herbage growth (Stone, 1988; Mitchell, 1990). It has long been recognised however that at extremes of the natural breeding season fecundity is reduced, a possible consequence of seasonal variations in ovulation rate, fertilisation and/or embryo survival (Hammond, 1944). Daylength is the factor which entrains the endogenous rhythm of reproduction in sheep (Donovan, O’Callaghan, Karsh, Boland and Roche, 1992) and daylength changes per se may be responsible for variations in reproductive response throughout the breeding season. Associated environmental and management factors may also be of importance. The objective of the present study was to determine the effect of management history and month of mating on ovulation and lambing rates in Mule ewes.