I compare the sizes of non-native and native ants to evaluate how worker si
ze may be related to the ability of a species to invade new habitats. I com
pare the size of 78 non-native ant species belonging to 26 genera with the
size of native congeneric species; native ants are larger than non-native a
nts in 22 of 26 genera. Ants were sorted by genera into fighting and nonfig
hting groups, based on observations of interspecific interactions with othe
r ant species. In all of the genera with monomorphic worker castes that fig
ht during competition, the non-native species were smaller than the native
species. The genera that engage in combat had a higher frequency of signifi
cantly smaller size in non-native ants. I selected Wasmannia auropunctata f
or Further studies, to compare native and non-native populations. Specimens
of W. auropunctata from nonnative populations were smaller than conspecifi
c counterparts from its native habitat. I consider hypotheses to explain wh
y non-native ants are smaller in size than native ants, including the role
of colony size in interspecific fights, changes in life history, the releas
e from intraspecific fighting, and climate. The discovery that fighting non
-natives are smaller than their closest native relatives may provide insigh
t into the mechanisms for success of non-native species, as well as the rol
e of worker size and colony size during interspecific competition.