Abstract:Elephant seals breed in Patagonia (Península Valdés, Argentina) from late August to early November, reaching peak numbers during the first week in October. Observations of this population over the past ten years yielded similar results. Eighty percent of the pups were born by 2 October. Most (96%) of 663 females marked during three breeding seasons gave birth to a pup. Females stayed on land a mean of 28 d, gave birth 6 d after arrival, nursed their pups for 22 d, and copulated a mean of 2.5 times 20 d after parturition and 2 d before departure. Copulations peaked during the third week in October. Males spent 57–80 d on land fasting and defending harems of up to 134 females (median 11–13 females, depending on year). Most (96%) marked females that gave birth (n= 636) also weaned their pups successfully. Pup sex ratio was unity. Harems were smaller and breeding occurred about three weeks earlier in Patagonia than in other colonies. Thermal conditions, day length and food availability may explain clines in the timing of breeding events between populations, Other parameters of the breeding season for the expanding Patagonia colony are similar to those for declining southern elephant seal populations else