Kleptoparasitism of Coots by Gadwalls was recorded at a southern Spanish lagoon during a year of low water level, but not in previous years, when water levels were higher. Female Gadwalls parasitized Coots more frequently than did males. Once the Coot surfaced, the Gadwalls waited a few seconds before starting to parasitize, which could serve to prevent eliciting an aggressive response from the Coot. The time that the Coots spent feeding on the water surface was similar irrespective of whether ago- nistic interactions were or were not involved, but the time period elapsing since the Coots stopped feeding until the next dive was longer when they responded aggres- sively. No apparent advantage in terms of food intake resulted for parasitic Gadwalls relative to birds feeding conventionally. The food intake of these ducks decreased when parasitizing more than one Coot. Parasitized Coots dived for food more often than unattended ones, hence compensating for the decrease of food intake caused by Gadwalls. It is hypothesized that kleptoparasitism occurred because of low quality of available food resulting from decreased water levels. Some social aspects of klepto- parasitism are discussed. A lack of aggressiveness by coots toward kleptoparasites has been reported by most authors, an observation contrasting with the aggressive habits characteristic of these species. A simple model is presented to explain this seeming paradox.