The International Space Station (ISS) IATCS (Internal Active Thermal Control System) includes two internal coolant loops that use an aqueous based coolant for heat transfer. A silver salt biocide was used initially as an additive in the coolant formulation to control the growth and proliferation of microorganisms in the coolant loops. Ground-based and in-flight testing has demonstrated that the silver salt is rapidly depleted and not effective as a long-term biocide. Efforts are now underway to select an alternate biocide for the IATCS coolant loop with greatly improved performance.An extensive evaluation of biocides was conducted to select several candidates for test trials. Selection criteria included: the need for safe, non-intrusive implementation and operation in a functioning system; the ability to control existing planktonic and biofilm residing microorganisms; a negligible impact on system-wetted materials of construction; and a negligible reactivity with existing coolant additives.Candidate testing is underway to provide data to select an optimal alternate biocide. That testing includes rapid biocide effectiveness screening using Biolog MT2 plates, minimum inhibitory concentration determination defined as the amount of test substance that will inhibit growth of microorganisms, materials compatibility evaluations, and coolant compatibility studies. In parallel with this effort, methods to introduce and maintain effective system biocide concentrations in-flight are being investigated.This effort will result in the selection of an optimal alternate biocide and delivery method that will subsequently undergo ground verification testing before implementation.This paper reports the current status of the effort to select an alternate biocide for the ISS IATCS coolant loops. The evaluation that was conducted to select candidates for test trials is presented.