Organisms that are abundant locally in a habitat patch are commonly observe
d to be frequent regionally, or among patches. In parasites, species presen
t in high numbers in host individuals are also present in many individuals
in the host population. On a larger scale, however, when host species are c
onsidered as patches, we may expect the opposite pattern because of the cos
t of producing mechanisms to evade the immune responses of several host spe
cies. Thus parasite species exploiting many host species may achieve lower
average abundance in their hosts than parasite species exploiting fewer hos
t species. This prediction was tested with data from 188 species of metazoa
n parasites of freshwater fish, using a comparative approach that controlle
d for study effort and phylogenetic influences. A negative correlation was
found between the number of host species used by parasites and their averag
e abundance in hosts, measured as either prevalence or intensity of infecti
on. There was no evidence that parasite species fall into distinct categori
es based on abundance patterns, but rather that they fall along a continuum
ranging from a generally low abundance in many host species, to a generall
y high abundance in few host species. These results applied to both ecto- a
nd endoparasites. The pattern observed suggests the existence of a trade-of
f between how many host species a parasite can exploit and how well it does
on average in those hosts.