1. Four nymphalid butterflies, Aglais urticae L., Inachis io L., Polygonia c-album L. and Vanessa atalanta L., share the same primary host plant, Urtica dioica L., but have different margins to their U.K. and European ranges. Their developmental responses to a series of constant temperatures were measured.

2. Degree-day requirements were found broadly to explain the relative distributions and differences in voltinism of A. urticae, P. c-album and I. io. The migrant V. atalanta did not fit into the predicted pattern, and this species may be more limited by its ability to overwinter.

3. Although the most northerly distributed species, A. urticae, had the lowest degree-day requirement, it had the highest developmental threshold and performed best (for mortality, pupal weight and relative growth rate) at high experimental temperatures. It is suggested that this may be due to the gregarious nature of its larvae and their possible ability to thermoregulate.

4. At southern margins, different distributional limits may be explained partly by larval gregariousness (a more northern trait) and maximum temperatures at which development is possible.

5. Limits to the distributions of these mobile species are at least partially explicable by the interaction of climatic patterns and thermal biology. A rapid response to climate change is predicted, and has already been observed in two of the species.