Common terns (Sterna hirundo) exhibit a remarkable range of variation in reproductive success. Several factors are known to contribute to reproductive failure either before hatching or between the time of hatching and fledging : predation pressure, food availability, flooding, competition for nesting sites, and toxic chemicals. Contaminants such as organochlorine pesticides, polychlorinated biphenyls (PCBs), polychlorodibenzodioxins (PCDDs), polychlorodibenzofurans (PCDFs), mercury and selenium were proved to significantly impair tern reproduction. During the reproductive period of 1988, an important mortality of common terns was observed in french colonies around the river Rhine. Approximately 50% of the young died a few days after hatching. The objective of the present study was to determine whether the intoxication by chlorinated compounds could have been responsible for the observed reproductive failure.