This study was conducted to evaluate the influence of animal and plant protein diets on sperm quality indices over 120 days, using the vervet monkey (Cercopithecus aethiops), as a model. These experiments were divided into a 60-day period of high-protein consumption (+/-17% crude protein), followed by a 60-day term of sustainable protein intake (+/-9% crude protein). All the diets were designed to be similar, except for the source of dietary protein that the animals consumed. High-protein diets containing milk solids or maize + legumes had no significant effect on sperm quality parameters over the first 60 days. During the next 60 days of the investigation, sustainable plant and animal protein diets had differential effects on a number of sperm quality indices. When compared to the plant-based diet, the monkeys that were given the animal protein diet containing milk solids had lower sperm counts (p < .04), reduced sperm motility (p = .04), higher sperm midpiece abnormalities (p < .05), and a trend (p = .10) towards increased sperm head defects. These findings shed some light on the impact of variable dietary proteins on sperm quality, but should be followed by longer-term investigations around this important reproductive health issue.