Trials were conducted aboard the tanker Seabulk Mariner to test a natural product, SeaKleen, as a biocide controlling non-indigenous populations of plankton and bacteria in ballast water. SeaKleen was dosed into matched ballast tanks at two different concentrations, 0.8 mg L(-1) active ingredient (a.i.) and 1.6 mg L(-1) a.i. during ballasting off the Oregon coast during a three-day passage to Prince William Sound, Alaska. Live organism counts from treated ballast water were compared with those from untreated (control tank) water collected from the same source location. Shipboard chemical analyses were made to verify dose and quantify chemical degradation and residuals following dilution. Results indicated that both SeaKleen doses resulted in complete zooplankton and phytoplankton mortality and that the higher dose (1.6 mg L(-1) a.i.) caused a two-log removal of culturable bacteria over a 92 h grow-out period. Spectrophotometry confirmed initial dosing to within 5% of nominal values. Shipboard bioassays were conducted using larval fish (Cyprinodon variegatus), brine shrimp (Artemia salina) and the bioluminescent dinoflagellate Pyrocystis lunula. Exposure of the test organisms to water drawn from treated ballast tanks 48 h after SeaKleen was added to the tanks resulted in 100% mortalities in Cyprinodon and Pyrocystis at both doses. Corresponding mortalities for Artemia larvae were 100% and 60% for high and low SeaKleen doses, respectively. Toxicity testing of treated water, subjected to varying dilutions, indicated that residual toxicity to even the most sensitive organisms would be eliminated once the discharge had dispersed beyond 100 feet from the vessel.