Aspects of the breeding biology of the world largest Olrog's Gull (Larus atlanticus) colony, in the estuary of Bahía Blanca, Argentina, were assessed for 101, 66, and 47 nests in 2005, 2006, and 2007, respectively. Mean (± SD) clutch size in 2005 was 1.86 ± 0.73 eggs per nest and modal clutch size was two eggs (range = 1-3). The incubation period was 1.67 days longer for A-eggs than for B-eggs (27.44 ± 1.22 days vs. 25.77 ± 1.36 days, respectively; P < 0.001). Incubation length for C-eggs was 25.75 ± 0.96 days. The largest eggs were 31.5% (length), 21.3% (breadth), and 66.5% (volume) larger than the smallest eggs. Mean egg volume in 2006 and 2007 decreased with hatching order, but the magnitude of this change was more pronounced in 2007 than in 2006. Variation in all egg measurements was larger among than within clutches. Hatching success within three-egg clutches was 76.9% in 2005, 81.7% in 2006, and 91.3% in 2007 (P = 0.20). Total egg loss in 2005 reached 16.7% and complete clutch loss was 43.8% during the incubation period. Parameters quantified in this study provide a comparative benchmark for future research on factors affecting breeding parameters in Olrog's Gull from this and other colonies, and lay the foundation for developing effective conservation strategies for the species.