Neonatal tetanus (NT) is a severe, often fatal disease caused by a toxin of Clostridium tetani, a ubiquitous spore-forming bacterium found in high concentrations in soil and animal excrements. NT is associated with nonsterile delivery and umbilical cord-care practices for newborns of mothers with antitoxin levels insufficient to protect the newborn by transplacental transfer of maternal antibody. In 1997, NT accounted for an estimated 277,400 deaths worldwide but is rare in the United States. During 1995-1997, of 124 tetanus cases reported in the United States, only one occurred in a neonate. This report summarizes the investigation in March 1998 of an NT case by the Missoula City-County Health Department (MCCHD) and the Montana Department of Health and Human Services (MDHHS). The findings indicated that tetanus in a newborn of an unvaccinated mother occurred after application of nonsterile clay to the umbilical cord.