Data for five closely related species of gammarid crustaceans are used to examine interspecific relationships between the breadth of fundamental tolerance or capacity and geographical range size. Gammarus duebeni is, almost without exception, the most tolerant species and that with the best physiological performance. Although there is some limited variation, the remaining species can be ranked broadly in the sequence G. zaddachi > G. salinus > G. oceanicus > G. locusta. The wide tolerance and high performance of G. duebeni is associated with the occupation of a wider range of environmental ‘types’ than any other of the species. In terms of geographical range size, the species can be ranked from most to least widespread in the sequence G. oceanicus > G. duebeni > G. zaddachi > G. salinus > G. locusta. This provides little support for Brown’s hypothesis, or the argument that the more widely distributed species within a taxonomic assemblage also tend to have the widest fundamental niches. However, if marine (G. oceanicus and G. locusta) and estuarine (G. duebeni, G. zaddachi, G. salinus) species are considered separately, then in each case the species with the largest geographical range is also the most tolerant/best performer. In this sense, the jack-of-all-trades is the master-of-all, rather than the master-of-none.