Male-male mountings were commonly performed by sexually monomorphic Razorbills Alca torda in mating arenas where males aggressively competed over extra-pair copulations (EPCs) with visiting females. In two breeding seasons, 417 male-male mountings were recorded. Forty-one percent of all non-pair mountings were of males, 66% of males performed male mountings, and 91% of males received mountings. Several lines of evidence suggest that male mountings were a form of fighting over EPCs rather than a case of mistaken identity: (1) In a multiple regression, the percentage of all mountings that were of males increased with the male-male aggression rate rather than with the male:female sex ratio; (2) the frequency of male mountings also increased with the aggression rate, rather than with the frequency of EPC attempts of females; (3) a large percentage (41%) of male mountings occurred after the mounted male had been settled in the arena and had time to be recognized, and not only immediately upon arrival, when mistaken identity may have been expected; and (4) males made significantly more, rather than fewer, male mountings as their age increased, suggesting that lack of experience did not lead to mis-identifications. The number of male mountings performed was positively correlated with EPC success, as well as with three other variables associated with EPC success, suggesting that male mounting belongs to a suite of behaviors aimed at obtaining extra-pair copulations. A possible benefit of this behavior is to inhibit competitors from attending the mating arenas, because there was a negative correlation between the attendance of males and the number of mountings they received per day. This is the first study to report quantitative evidence that male-male mounting by birds is beneficial.