Guard hairs of mammals are useful in identifying species. Morphological characteristics of the guard hairs were used to determine differences in the pelage of mona monkeys (Cercopithecus mona) in Omo-Shasha-Oluwa Forest Reserves in South western Nigeria. Using standard procedures, five strands of guard hair from each location were observed under a light microscope attached with a digital camera for the determination of histological structures. Qualitative and quantitative characteristics of the hair were examined and data obtained directly from the microscope. Data on the cuticle, cortex and medullar dimensions (μm) of the hair were analysed descriptively and inferentially using SPSS Version 20 respectively. Variance analysis was used to compare the means of the quantitative parameters of the hair. Statistically significant means (P ≤ 0.05) were separated using LSD post hoc test. A continuous medulla pattern was common in all locations. Amorphous medulla was found in only Omo and Shasha monkeys. The longest scale length and width of 218.76 ± 60.29μm and 725.76 ± 155.91μm respectively were recorded in Omo Forest Reserve. Medullary diametre, index, and a fraction of monkeys in Omo and Shasha were significantly different (P ≤ 0.05) from that of Oluwa. Based on the qualitative and quantitative characteristics of the guard hair, the mona monkeys in Omo and Shasha had features that were similar. The findings can be useful in explaining the close proximities and interactions of this species in the three Reserves. The information can be used as a guide in conservation, forensic and other scientific researches.