The presence of a major gene influencing prolificacy has been proposed for the Cambridge breed. Cambridge ewes of extreme phenotype were identified by ovulation rate (OR) and litter size (LS) recorded in a single flock from 1976-1994. Means (sd) for all observations for OR (n=2062) and LS (n=2480) were 3.0 (1.69) and 2.5 (1.02). Extreme ewes were identified by three criteria: (a) High prolificacy had at least one OR observation ≥6; (b) Low prolificacy had records in at least three years with no OR or LS record >3; (c) Ewes had progeny with records in at least three years. On the basis of these criteria 27 High and 67 Low ewes were identified with 1.6 and 1.5 progeny per ewe, 2.1 and 2.5 OR records per progeny, and 3.7 and 1.7 LS records per progeny. The OR and LS results for ewes and their progeny are shown in Table 1. The maximum observed OR of High ewes ranged from 6-13 and both the OR and LS of Low ewes had a mean of 2.2 and ranged from 1-3. Means (se) of the progeny of the High and Low groups (determined in a model that included dam group, progeny within group, year and progeny age) were 3.9 (0.19) and 3.0 (0.13) for OR (P=0.003), and 2.6 (0.12) and 2.4 (0.11) for LS (P=0.055).