After weaning, dairy calves are commonly exposed to changing social environments, which are likely to influence activity patterns. We examined the activity of recently weaned dairy calves upon introduction to a social group. Holstein heifer calves (n = 26) were individually housed and provided milk (10 L/d) for 6 wk, and gradually weaned over 9 d. Calves were then introduced to group housing on pasture (15.2 × 45.7 m) in weekly cohorts (6 ± 2 new calves/wk; mean ± SD) at 60 ± 3 d of age. The group (9 ± 1 calves) always consisted of newly introduced calves and calves introduced the previous week. Calves were equipped with electronic data loggers (HOBO Pendant G data logger, Onset Computer Corp., Pocasset, MA) to measure daily standing time and bout frequency for a 2-wk observation period: the first week following grouping, and the second week, which began with addition of younger calves and removal of older calves. The effects of week and day on daily standing time and bout frequency were analyzed in a double repeated measures general linear mixed model. Additionally, standing time was summarized by hour to assess diurnal activity patterns. Activity data were subject to a week by day interaction (P < 0.003), with a peak on the day of introduction for standing time (8.8 vs. 7.1 min/d; d 1 of wk 1 vs. d 1 of wk 2; SE = 0.42; P = 0.0046) and bouts (20.5 vs 13.6 bouts/d; SE = 1.35; P < 0.001) but decreasing activity over wk 1 and stable activity during wk 2. Newly introduced calves also stood longer in the hour after introduction but spent less time standing later in the day, compared with their diurnal activity on the first day of the following week when younger calves were added (week by hour interaction; P < 0.001). These results suggest that the first few days of introduction to a social group have a marked influence on activity patterns, but that previously grouped calves are less responsive to the addition of new calves into the group.