Mitochondrial DNA (mtDNA) sequence data were used to compare the popul
ation genetic structures of five species of parasitic nematodes from t
hree different hosts: Ostertagia ostertagi and Haemonchus placei from
cattle, H. contortus and Teladorsagia circumcincta from sheep, and Maz
amastrongylus odocoilei from white-tailed deer. The parasites of sheep
and cattle showed a pattern consistent with high gene flow among popu
lations. The parasite of deer showed a pattern of substantial populati
on subdivision and isolation by distance. It appears that host movemen
t is an important determinant of population genetic structure in these
nematodes. High gene flow in the parasites of livestock also indicate
s great opportunity for the spread of rare alleles that confer resista
nce to anthelmintic drugs. All species, including the parasite of deer
, had unusually high within-population diversities (averages of 0.019-
0.027 substitutions per site between pairs of individuals from the sam
e population). Large effective population sizes (Ne), perhaps in combi
nation with rapid mtDNA evolution, appear to be the most likely explan
ation for these high within-population diversities.