Analysis of sediment samples collected in Lake Ontario in 1968 revealed the occurrence of mirex in two anomalous zones related to input from the Niagara and Oswego Rivers. These anomalies were confirmed in a 1976 resampling program. A suspended solids sample taken in the Niagara River confirmed an upstream source (Hooker Chemical) of the compound. Bottom sediment samples in the Oswego River identified an industrial source (Armstrong Cork) 14 km upstream of the river mouth. Known use of mirex by this plant indicated a substantial loss some 15 years ago. This was confirmed by sediment coring in the open lake sediments indicating that mirex deposition commenced 7 to 14 years before present.