In a study on size and reproductive behaviour in the southern elephant seal,Mirounga leonina, on South Georgia from 1991 to 1993, large size, measured as nose to hind flipper length, was associated with factors that confer an advantage in terms of reproductive success. Some males stayed at the centre of harems, others were on the periphery and others were outside the harem. A male’s mating success increased with his length, and for central males, mating success increased with harem size. Central males defeated almost all other males. Central males were also longest and outside males shortest. Furthermore, central males stayed ashore longest. For central males, length was positively correlated with number of days spent in the harem but not with harem size. Central and peripheral males had the highest copulation frequencies (per h and per male). The copulation frequency of central males apparently peaked in harems with a maximum size of 100–150 females, with lower frequencies in larger and smaller harems. However, the mating success of central males over the season increased with harem size. The rate of alert behaviour and time devoted to resting were not correlated with harem size for any of the three male categories, but rate of agonistic behaviour increased with increasing harem size for central males.