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This paper describes the identification of a common mold (i.e., fungus) growing on nonceramic insulators (NCIs) in service, and analyzes its impact on the electrical performance. The insulators evaluated were line posts removed from a 138-kV transmission line serving the interior (away from the coast) regions in Florida. The insulators had housings made from different types of silicone rubber (SR) and ethylene propylene rubber (EPR) polymer families. Electrical tests performed include surface resistance measurements and determination of contamination withstand capability (CWC) using the clean-fog procedure. Changes in the CWC were correlated to material changes by using micro-Fourier transform infrared (FTIR) analysis and surface resistance measurements. It was concluded that the mold growth is dependent on material formulation and outdoor environment. On SR insulators that exhibited mold growth, there was a reduction in the CWC when compared to the same insulator without the mold; however, even the reduced level of CWC was superior to that obtained on similarly rated EPR and porcelain insulators without any visible mold growth. Lastly, simple methods to remove the mold from the insulators are listed. [ABSTRACT FROM AUTHOR]