A comparative study of grazing behaviour was conducted on temperate pasture between June and October 1989, using 20 high (H) and 20 low (L) merit Friesian cows. Mean daily milk yields were 32.0 kg/day for H cows and 24.8 kg/day for L cows, based on the record of previous weeks. The experimental animals were managed within a large herd and grazed on perennial ryegrass swards in a 21-day rotational-grazing system. The observations were taken during the period between morning and evening milkings (approx. 7 h). During the study period, drought conditions occurred, leading to grass scarcity and the need for supplementary feeding Total grazing time (TGT) and the duration of the first grazing bout (FGB) were significantly higher for H merit cows (P<0.001 and <0.05, respectively). There was a significant relationship between lactation number and both TGT (P<0.05) and FGB (P<0.001); grazing time tended to decline as lactation number increased. No interaction was found between milk yield and observation date for TGT, but a significant interaction occurred for FGB (P<0.001). Biting rate was higher for H than for L groups (63.5 and 60.8 bites/min, respectively, P<0.05) and decreased as the grazing bout progressed (r2 = 0.20, P<0.001).The relative level of movement by cows while grazing was not related to their milk yield but it was related to lactation number as younger cows moved further than older ones (r2 = 0.68, P<0.05). Distribution of H and L cows over the paddock was similar during grazing.