Attempts at intraspecific kleptoparasitism by Lesser Black-backed Gulls (Larus fuscus) feeding at a rubbish dump in southwest Spain during the winter 2001-2002 were studied to measure differences in the frequency of attacks between the age classes, the success of the attacks and the existence of possible density-dependent kleptoparasitism. Significant age differences in the frequency of attacks initiated were not found, although first year birds received a higher number of attacks. The success of the attacks increased with the age of the kleptoparasite and decreased with the age of the victim. The gulls selected victims of an equal or younger age to their own. A correlation was found between density and number of attacks on most dates. The possibility exists that kleptoparasitism could influence the success of the recent expansion of the species in the Iberian Peninsula.