In organic farming, the production of pork from entire male pigs seems to be the most promising alternative to castration, as organic animal husbandry aims at high standards of animal welfare and elimination of mutilations. However, previous research on behaviour of entire male pigs was mostly carried out under barren housing conditions and is thus not entirely transferrable to enriched housing conditions. In the present study, we investigated single-sex and mixed-sex groups of entire male pigs and compared them with mixed-sex groups of castrated male pigs under organic housing conditions. On a commercial farm, 362 pigs (average weight 22.7 kg, average age 80 days) were assigned to pens with groups of 20 (+1) pigs according to three treatments: single-sex groups with entire males (EE), mixed-sex groups with entire males and females (EF) and mixed-sex groups with castrated males and females (CF). Skin and tail lesions, lameness and the behaviours ‘head knocking/biting’, ‘fighting’ and ‘mounting’ were assessed on five days during the fattening period. Average carcass weight for all sexes was 92 kg. After slaughter, female reproductive organs were inspected for signs of pregnancy. Fat samples of entire male pigs were analysed for concentrations of androstenone and skatole. Entire males showed more aggressive and mounting behaviours than castrated males (p < 0.001), and no difference was found between entire males from the EE and EF treatments. Welfare of female pigs was not impaired when penned together with entire males compared to when penned together with castrates, neither in terms of skin lesions nor in terms of received aggressive behaviour. Pigs in the EE treatment had higher lesion scores than those in the EF and CF treatments (p < 0.05 and p < 0.01, respectively), while the two latter treatments did not differ. In the inspected female reproductive organs, no pregnancy could be detected. The concentration of androstenone in fat indicated that the presence or absence of females in a group did not have an effect on the male pigs. Of the 177 slaughtered entire male pigs, only one carcass was classified as containing boar taint by a human nose test. Our findings suggest that under enriched housing conditions, entire males can be kept in single–sex groups as well as in mixed-sex groups with females without compromise in animal welfare.