we studied the breeding biology of the american oystercatcher (haematopus palliatus) during four breeding seasons (1997-2001) at mar chiquita coastal lagoon reserve, mar de cobo and la caleta (southeastern buenos aires province, argentina). nests were laid in habitats with low vegetation cover and high stone density, close to the dunes. the initial egg laying period (september-october) lasted 27 days. tw o replacement clutches were recorded. hatchings were recorded from october until december. there were no differences between the sizes of first and second clutches. we recorded a negative correlation between clutch size and laying date. breeding success was low, with a hatching success of 25.85% and 0.24 fledging per pair per year. there was a negative relationship between breeding success and laying date. egg losses were due to severe storms, predation and human disturbances. clutch size and egg volume were lower than those reported for the subspecies haematopus palliatus palliatus from north america, whereas breeding success was similar. due to the low breeding success recorded and the effects of human disturbances we propose to restrict the use of beaches by people during the breeding season.