Murine norovirus (MNV) causes subclinical chronic infections in adult immunocompetent mice and is endemic in many mouse colonies. The susceptibility of neonatal mice to MNV infection was investigated. Intestinal homogenates from Swiss Webster (SW) mice inoculated orally with MNV-L on postpartum days (ppd) 1 to 3 were negative for MNV by RT-PCR at postinoculation days (pid) 3 and 7. In contrast, 69% of intestinal homogenates prepared on pid 3 and 7 from mice inoculated orally at ppd 5 to 8 were MNV-positive by RT-PCR. Because only mice 10 d of age or older were infected by contact with infected dams, a study was performed to determine whether fostering of neonatal mice from MNV-infected to MNV-naïve dams could be effective at preventing infection of neonatal mice. Four litters each of 1-, 2-, 4-, or 6 d-old mice from MNV-L- infected dams were transferred to naïve dams with similar-aged litters and vice versa. On ppd 21, feces from all MNV-infected dams and litters transferred to them were MNV-positive. In contrast on ppd 21, feces from all MNV-naïve dams and litters transferred to them were MNV-negative. Fostering of 2-d-old mice from 5 of 5 MNV-C-, 5 of 6 MNV-D-, and 7 of 8 MNV-G- infected dams onto MNV-naïve dams prevented MNV infection of the foster mice. In the 2 litters where MNV was detected, dams were infected within 7 d of transfer, suggesting that the neonatal mice had served as fomites. In summary, fostering was effective at preventing MNV infection in 33 of 35 litters of neonatal mice. Precautions to prevent transmission of virus on the surface of neonatal mice to foster dams could increase the efficiency of the fostering process.