A model of persistent gestational mumps virus infection has been developed in the chicken. Virus, inoculated into the yolk sac at 12 hours, persisted throughout fetal life and for a few days following hatch. Although virus was widely distributed in the fetus, inflammatory change occurred only in the heart. Myocarditis was detected initially at 18 days of fetal life, was most impressive at hatch, and resolved within 1 week after hatch. Examination of these chickens at 1 year of age revealed histological evidence of endocardial fibroelastosis.