As part of a study comparing grazing patterns of cows previously observed on steep and high terrain (hill climbers) and gentle terrain near water (bottom dwellers), social interactions were observed in 4 foothill pastures during the summers of 2000 and 2001.  In each pasture, cows were observed on 10 to 13 occasions.  Group size was defined as cows within 30 m of each other and separated by more than 60 m.  Herd size varied 27 to 111 cows.  Average and maximum group size were related to herd size (P ≤ 0.05).  Maximum and average group size was 22 ± 4 and 8 ± 1 animals (mean ± SE), respectively, when herd size was less than 30 and 11 ± 1 and 4 ± 1 when herd size was greater than 30.  Bottom dweller cows had larger average and maximum group sizes than hill climbers (P < 0.01).  Average group size was 5.6 ± 0.3 for bottom dwellers and 4.5 ± 0.3 for hill climbers, while maximum group size was 14.9 ± 0.9 and 11.3 ± 0.8, respectively.  In addition, evaluations of 42 randomly-selected cows were used to determine if 2 or more cows were in the same group during multiple observations.  Cows were alone during 22 ± 1% of observations.  Over 75% of cows were observed in the same group as 1 or more cows on 2 observations.  However, no cows were observed with the same cow on 5 or more observations when herd size was greater than 50.   In contrast, over 80% of cows were observed with the same cow in the same group during 5 observations when herd size was less than 30.    These data suggest that fidelity to social groups may depend on herd size.  Research examining cattle social interactions should consider herd size.