The relationships between regional species richness and local species richness were examined in respect of helminth parasite communities in 32 species of freshwater fish in the British Isles. Fish were divided into 5 categories, for each of which the goodness of fit of the relationship to linear, exponential and power function models was tested. For all categories of fish combined, there was a significant, positive curvilinear relationship. Nested within this were two other patterns. For introduced fish, a linear model provided the best fit; for euryhaline and relict species it was impossible to determine the best model, but for the other categories the relationship was curvilinear and was best fitted by a power function model. The linear relationship found for introduced fish was interpreted as a temporary situation, reflecting the shortage of time for the communities to become saturated. It corresponded to the linear part of the curvilinear relationship of the other categories, which is believed to represent the fundamental form of the relationship for parasite communities. The communities reached a saturation level of richness, corresponding to the asymptote of the curve, which fell well below regional species richness. Explanations for local saturation are discussed, but neither community structure nor supply-side ecology can yet be preferred. It is concluded that local patterns in helminth community richness, in contrast to those in fish assemblages, are not significantly influenced by patterns on a larger, regional spatial scale and so regional species richness is not a key determinant of local species richness, nor does a knowledge of regional patterns improve predictability of local patterns.