The weaning process presents a significant challenge in pig production. It involves the early separation of piglets from sows, resulting in an abrupt transition from milk to specially formulated feed. This complex combination of different litters into a new group, where pigs have limited space and opportunities for constant interaction, prevents them from fully displaying natural agonistic behaviors necessary for establishing a social hierarchy. This study aimed to explore how spontaneous mixing of piglets affects aggressive behavior during the weaning process. Video cameras were used to document two pens in the breeding facility, and direct observation was employed to assess the frequency and duration of suckling as well as the number of interactions among piglets, including aggressive behavior and play. It was observed in Pen I that piglets from the same litter were less aggressive towards each other compared to piglets from different litters. Aggression was recorded 36 times among piglets from the same litter and 62 times among piglets from different litters. Compared to pen I, in pen II, there was no significant difference in aggressive behavior between piglets from the same litter and piglets from different litters after mixing. No significant differences were detected in attack duration between resident and mixed liiters. More attacks were recorded at the beginning of the mixing, specifically during the first two days after mixing.