An experiment was carried out from August to early November 1994 to examine differences in diet selection, herbage intake, grazing behaviour and animal performance between weaned lambs rotationally grazing swards of annual ryegrass (Lolium multiflorum)/white clover (Trifolium repens) and Yorkshire fog (Holcus lanatus)/T. repens with or without Lotus corniculatus. There were four replicate groups of six lambs per treatment. The effects of condensed tannins (CT) on lamb production were assessed by twice-daily oral administration of 10g polyethylene glycol (PEG; molecular weight 4000) to half the lambs on each sward. The Lotus content of all swards was very low, and results are presented here for main sward comparisons meaned over lotus treatments. Overall mean estimates of pre-grazing herbage mass and sward surface height for the annual ryegrass and Yorkshire fog swards respectively, were 5820 v. 4360±190 kgDM/ha (P<0·001) and 29 v. 21±0·6cm (P<0·001). The coefficient of organic matter digestibility (OMD) of the diet selected and herbage intake were higher on Yorkshire fog than on annual ryegrass (0·78 v. 0·74 ± 0·080g/kg; P<0·05, and 1070 v. 860±57g OM per lamb per day, P<0·05 respectively), reflecting the higher content in the diet of grass green leaf (980 v. 930g/kg±14g/kg, P<0·05) and the lower content of dead material (80 v. 110±15g/kg, P<0·08). Lambs grazing on Yorkshire fog swards had higher clean wool growth rate (1470 v. 1280±30mg/cm per day, P<0·01) and greater fibre diameter (31 v. 29±0·2μ, P<0·001), greater liveweight gain (152 v. 108±5·5g/day, P<0·001), final weight (42 v. 38±0·5kg, P<0·001), carcass weight gain (89 v. 69±2·5g/day, P<0·001), carcass weight (19 v. 17±0·3kg, P<0·001) and soft tissue thickness (GR value 11 v. 8±0·5mm, P<0·01), and lower faecal egg counts (FEC; square root transferred values 9·2 v. 11·0±0·4 eggs/g fresh faeces, P<0·01) than lambs grazing annual ryegrass swards. Similar dietary concentrations of condensed tannins (CT) between Yorkshire fog and annual ryegrass swards (4·2 v. 3·7 DM±0·2g/kg, P<0·08) increased clean wool growth (1440 v. 1310±32 mg/cm per day, P<0·05), fibre diameter (30·7 v. 29·5±0·21μ, P<0·01) and liveweight gain (141 v. 120±4·3g per lamb per day, P<0·01), although differences in carcass weight (17·9 v. 18·2±0·3kg) and FEC transformed values (9·6 v. 11·0±0·6 eggs/g fresh faeces) were not significant. The effects of CT on animal performance were greater in Yorkshire fog swards. CT had no significant effects on diet selection, herbage intake and grazing behaviour patterns.