Olrog's Gull Larus atlanticus is a vulnerable species endemic to the Argentine Atlantic coast. We present information on new breeding colonies, update information on known colonies, and describe habitat and nest site characteristics of Olrog's Gulls breeding at Bahía San Blas, southern Buenos Aires Province, Argentina. Information was obtained during the 1998 breeding season. We recorded four colonies on islets, with a total population size of 305 pairs. Colonies were located on open ground, partly bordered by vegetation. The two colonies at Islote Arroyo Jabalí Oeste were located more than 150 m from the high tide line, while the two colonies at Banco Nordeste were placed only a few metres from the high tide mark (1-6 m). Most nests were located more than a metre away from low Atriplex patagonica bushes (20-30 cm high) and herbaceous species, although a few nests were a few cm from vegetation. Nests were built on vegetation debris, on low herbaceous vegetation, and on gravel and sand. All nests at the four colonies were built with vegetation debris, mainly Spartina densiflora and some Salicornia sp. Mean external and internal diameters of nests were 40.57 ± 5.14 and 19.71 ± 2.21 cm, respectively. Gulls nested in discrete groups at relatively high densities, which ranged between 0.62 and 1.1 nests/m2. Nest occupation and number of eggs per nest varied among colonies. Kelp Gulls Larus dominicanus were observed nesting in close proximity to all colonies. Further studies are needed to assess habitat partitioning between these two species, particularly considering that they nest in association at all breeding locations in coastal Argentina and that Kelp Gulls have shown an important population increase in recent decades.