Summary: The causative agent of tularemia, Francisella tularen-
sis, is a formidable biologic agent that occurs naturally
throughout North America. We examined genetic and spa-
tial diversity patterns among 161 US F. tularensis isolates
by using a 24-marker multiple-locus variable-number tan-
dem repeat analysis (MLVA) system. MLVA identified 126
unique genotypes. Phylogenetic analyses showed patterns
similar to recently reported global-scale analyses. We
observed clustering by subspecies, low genetic diversity
within F. tularensis subsp. holarctica, and division of F.
tularensis subsp. tularensis into 2 distinct subpopulations:
A.I. and A.II. The 2 F. tularensis subsp. tularensis subpop-
ulations also represent geographically distinct groups; A.I.
occurs primarily in the central United States, and A.II.
occurs primarily in the western United States. These spatial
distributions are correlated with geographic ranges of par-
ticular vectors, hosts of tularemia, and abiotic factors.
These correlates provide testable hypotheses regarding
ecologic factors associated with maintaining tularemia foci.
Tularemia, also known as rabbit fever or deer-fly fever,