Abstract. Activity behaviour and social interactions of pigs in indoor and outdoor systems fed different levels of amino acids were studied on 96 crossbred pigs (Hampshire × Swedish Landrace × Swedish Yorkshire). The pigs were born outdoors and raised indoors in conventional pens or outdoors on pastures and given recommended (R), 7 % lower (R-7) or 14 % lower (R-14) levels of amino acids, in a phase feeding system with a low-energy diet provided ad libitum. Pigs in the outdoor system walked significantly more (p=0.012) and tended to be rooting more (p=0.098) than indoor pigs. Amino acid level did not affect the activity behaviour and social interactions of the pigs. Indoor pigs given diets R-7 and R-14 were drinking significantly more often than indoor pigs receiving the R diet, but had less contact with other pigs (p=0.020 and p=0.002, respectively). For outdoor pigs no such effect of amino acid level was found. Queuing for feed decreased with increasing age of the pigs, both indoors and outdoors (p=0.009). Rooting decreased and sleeping increased with the age of indoor pigs (p=0.014 and p=0.001, respectively), whereas no consistent trend for outdoor pigs was found. Sniffing, nibbling, pushing (p=0.001 for all) and tail manipulation (p=0.002) occurred more often indoors than outdoors. The results show that pigs in an outdoor system are more active and perform more natural behaviours, such as foraging and rooting, than pigs in an indoor system. Furthermore, roughages, such as pasture, and large areas may play an important role in occupying pigs and therefore contribute to less aggressive behaviours.