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Nucleoside analogues (NA) disrupt RNA viral RNA-dependent RNA polymerase (RdRP) function and fidelity for multiple viral families. The mechanism of action (MOA) of T-705 has been attributed alternatively or concurrently to chain termination and lethal mutagenesis depending on the viral species during in vitro studies. In this study, we evaluated the effect of T-705 on the viral population in non-human primates (NHPs) after challenge with Ebola virus (EBOV) or Marburg virus (MARV) to identify the predominant in vivo MOA. We used common virological assays in conjunction with deep sequencing to characterize T-705 effects. T-705 exhibited antiviral activity that was associated with a reduction in specific infectivity and an accumulation of low frequency nucleotide variants in plasma samples collected day 7 post infection. Stranded analysis of deep sequencing data to identify chain termination demonstrated no change in the transcriptional gradient in negative stranded viral reads and minimal changes in positive stranded viral reads in T-705 treated animals, questioning as a MOA in vivo. These findings indicate that lethal mutagenesis is a MOA of T-705 that may serve as an indication of therapeutic activity of NAs for evaluation in clinical settings. This study expands our understanding of MOAs of these compounds for the Filovirus family and provides further evidence that lethal mutagenesis could be a preponderant MOA for this class of therapeutic compounds.