A survey of five marinas and one reference site on Lake Ontario, Canada, was conducted to investigate the occurrence and variation of tributyltin (TBT) levels in water between April and December of 1998. The survey results revealed the presence as well as seasonal trends of TBT in these marinas. The TBT concentrations in water were found to coincide with boating activity. The level of TBT generally rose in summer when the boating activity was high, and the highest level of TBT found was 14 ng Sn/L. On average, 92% of the TBT was in the operationally defined “dissolved phase” in the waters analyzed. The butyltin compounds were also found in surface sediment and paint chips from some non-aluminum hulled pleasure boats, with total concentrations of butyltin species (including dibutyltin and monobutyltin) ranging from 59 to 570 ng Sn/g dry weight, and from 0 to 146 μg Sn/g, respectively. The results clearly indicated that TBT in the marina waters primarily originated from antifouling paint on pleasure boats, even though the use of TBT as an antifouling agent was regulated in 1989.