Post-copulatory sexual selection is theorized to favor female traits that allow them to control sperm use and fertilization, leading to the prediction that female reproductive traits that influence sperm migration should differ between polyandrous and monogamous species. Here we exploit natural variation in the female mating strategies of closely related Peromyscus mice to compare female traits that influence sperm motility – the viscosity, pH, and calcium concentration ¬of fluids in the reproductive tract – between polyandrous and monogamous species. We find that the viscosity and pH, but not calcium concentration, of fluids collected from both the uterus and the oviduct significantly differ between species based on mating system. Our results demonstrate the existence of a viscosity gradient within the female reproductive tract that increases in monogamous species but decreases in polyandrous species. Both species have a more alkaline environment in the uterus than oviduct, but only in the polyandrous species did we observe a decrease in calcium in the distal end of the tract. These results suggest that fluid viscosity and pH in the female reproductive tracts of these mice may be influenced by post-copulatory sexual selection and provide a promising potential mechanism for female sperm control given their importance in modulating sperm behavior.