Listeria monocytogenes is an endemic agent in the primate population at the California National Primate Research Center and has been associated with both sporadic cases and a general outbreak of pregnancy failures. The primary objective of this study was to verify the incidence of L. monocytogenes-associated abortion and fetal deaths in the Center's outdoor breeding colony. In addition, we sought to compare the group of female macaques that presented with Listeria-associated abortion with both those with nonlisteria-associated abortion and animals with successful pregnancy outcome. We calculated the incidence of L. monocytogenes-associated abortion and stillbirth by dividing the number of positive L. monocytogenes cultures from aborted fetuses by the number of pregnant female macaques from 1989 through 2009. To compare the pregnancy outcome of female macaques that have presented L. monocytogenes-associated abortion and stillbirth, we created 2 control groups: female macaques with successful pregnancy outcomes during the 1999 breeding season and animals with nonlisteria-associated pregnancy failure. These macaques were followed for 2 subsequent breeding seasons. The results showed a range in the incidence of L. monocytogenes-associated abortion and stillbirth from 0% to 8.39% throughout the 1989 to 2009 breeding seasons. In addition, the Listeria-associated abortion group did not present statistically significant differences in fertility and abortion rates when compared with the control groups. We conclude that although L. monocytogenes is an endemic agent at the Center's outdoor breeding colony, the agent's incidence varied in significance. Furthermore, an episode of L. monocytogenes-associated abortion did not affect subsequent pregnancies.