The breeding biology of the Slender-billed Gull (Larus genei) is poorly known. Several reproductive parameters have been recorded during ten years (from 1992 to 2001) at the Ebro Delta (northwestern Mediterranean), such as clutch size, egg size and breeding success, as well as the number of breeders during the study and their population dynamics. Average clutch size during the study was 2.57 eggs per nest, with three eggs being the modal size. Slender-billed Gulls can lay a second clutch after egg loss, although clutch size is decreased. Mean egg size for the period 1998-2001 was 38.50 ml (SE 0.11). Although breeding is highly synchronous, females laying first were probably those in best body condition. Average breeding success for the whole period was 0.71 chicks per nest (24% of eggs laid). Compared with other colonies, results suggest that on average, the site may be a high quality colony in terms of availability of food, nest sites and predation rates. However, most of the breeding parameters analyzed showed statistically significant differences between years, and also between breeding areas and sub-colonies within the delta. The number of breeding pairs increased slightly from 388 in 1992 to 468 in 2001 (mean annual growth rate was 1.02 per year), although the rate fluctuated greatly between years, these changes were probably related to emigration-immigration processes at a metapopulation level.