Studies were made of populations of adult Rhyssa persuasoria (L.) from various geographical regions. Morphometric analyses were based on the relative size of 32 maculae on various sclerites and the colour of the coxae and legs. Size was determined by measuring the wing length and thorax length. Various statistical procedures were applied to these data and also the ratios of wing length: maculation, and thorax length: maculation in an attempt to distinguish races of R. persuasoria from different localities. Regression analysis, analysis of variance and classification procedures separated some populations from others, the results of the different analyses being in broad agreement with each other. Within each population, maculation was typically linear, smaller specimens having relatively small maculae. Some sexually dimorphic differences were also demonstrated. There were two major geographical groupings: S.W. European and North African (W. France, Spain, Portugal and Morocco) and the remainder. There were some geographical differences within the remaining populations although all had links with one or more adjacent populations. The degree of maculation was shown to be related to the colour and texture of host-tree species, populations with low maculation scores being associated with tree species having dark, fissured bark and vice versa. It is suggested that degree of maculation is an expression of cryptic colouration, providing camouflage from predators. Determinations of heat-coma temperature for male R. persuasoria from climatically different areas showed that the response to high temperature differed between populations and was correlated with the temperature of the locality of origin. These differences between strains were apparent after several months of identical conditioning and are probably genetically based.