Due to successful conservation efforts, several marine mammal populations have increased in the past 40 years, leading to the possibility of increased competition for resources. We evaluated competition for haul-out space in western Atlantic harbor seals (Phoca vitulina concolor) by assessing agonistic interactions at 2 haul-out sites in Casco Bay, Maine. We predicted that intensity and rate of interactions increase as density increases, larger seals win more interactions than smaller seals do, seals already occupying a space win more interactions than arriving seals, and intensity of interactions is higher during molt but rates are higher during post molt. During molt at Gunpoint Ledge, 1 of the 2 sites, intensity of interactions decreased as density increased, but the pattern reversed during post molt. Rate of interactions peaked at intermediate density. Size and age class did not affect outcomes of interactions, but seals that already occupied space won more interactions than expected by chance. Interaction rate was higher during molt versus post-molt seasons. Harbor seals may experience increased competition for limited haul-out space as populations continue to grow, with seals occupying space outcompeting intruders.