 Using data from 12 U.S. health departments, we estimated mean serial interval for monkeypox virus infection to be 8.5, 95% credible interval 7.3 to 9.9, days for symptom onset, based on 57 case pairs. Mean estimated incubation period was 5.6, 95% credible interval 4.3 to 7.8, days for symptom onset, based on 35 case pairs. This article was authored by Zachary J. Madewell, Kelly Charniga, Nina B. Masters and others.