We investigated the identity of 37 Shigella flexneri strains that had previously been isolated from northern rural Vietnam (Son Tay Province) and described as untypable. Twenty-four isolates reacted with MASF 1c, a monoclonal antibody specific for S. flexneri serotype 1c. A further ten untypable isolates were found to be rough mutants (no longer expressing O-antigen) that were derived from serotype 1c strains. Pulsed-field gel electrophoresis demonstrated that these strains consisted of many different clones, indicating serotype 1c was well established in this region in the late 1990s. Serotype 1c was the most prevalent S. flexneri serotype isolated in the Son Tay Province, accounting for about 40% of S. flexneri isolates. Subsequent isolation of S. flexneri serotype 1c in this region and elsewhere in Vietnam confirmed that serotype 1c is of genuine importance in Vietnam.