This Great Ships Initiative (GSI) technical report describes outcomes from one shipboard status test evaluating the biological effectiveness and residual toxicity of a ballast water treatment system (BWTS) with applicability to U.S. flag vessels in Great Lakes trade. The installation tested was a temporary and partial (two tank) prototype installed in two tanks on board the motor vessel (MV) Indiana Harbor. The subject BWTS involved elevating pH by adding sodium hydroxide (NaOH, in the same formulation used for lye or caustic soda), retaining treated ballast water for a minimum period, and then neutralizing the ballast water prior to discharge. The test began on August 18, 2011, during normal vessel ballast intake operations in the port of Gary, Indiana, and concluded three days later on August 22 during normal vessel ballast discharge operations in the port of Superior, Wisconsin. In this single trial, BWTS-treated discharge contained live organisms ? 50 ?m (i.e., zooplankton) in concentrations ranging 178/m3 to 441/m3. These concentrations are lower than control discharge densities which ranged from 100,000/m3 to 167,000/m3. Densities of live organisms ? 10 and < 50 ?m in the treatment discharge ranged from 2 cell/mL to 8 cells/mL, while control discharge concentrations were higher, ranging from 53 cells/mL to 92 cells/mL. In terms of organisms < 10 ?m, the trial produced inconclusive results with concentrations of both total coliforms and heterotrophic bacteria highest in discharge samples from one of the treatment tanks. The results from a WET test indicate possible residual toxicity associated with one of the test species. The BWTS developer asserts that this toxicity could derive from artifactual pHdrift during the WET test; pH increased by a maximum of about one unit over the 24 hour period following each daily renewal. Overall, as a single replicate, this GSI status test of the prototype BWTS is in no way conclusive or determinative. Results provide only an indication of the system’s potential effectiveness relative to no treatment.