Plumage colouration, using quantitative methods, and size of 320 museum specimens ofButeo buteofrom Sweden were measured. The frequency of extremely pale birds was significantly higher in south Sweden than elsewhere. The amount of reddish colour in the plumage was minimal in central Sweden, rising northwards probably because of connexion with more easterly populations, and southwards probably because reddish pigments to a disproportionate degree persist even after an advanced general depigmentation. Size decreases clinally from south to north and a significant negative correlation was established between size and redness. No trace was found of any demarcation line between two distinguishable subpopulations, nor was there any bimodality in the data collected, and therefore nomenclatorial separation of the north Swedish population seems unjustified. Thus, the western limit ofButeo b. vulpinusmust be located east of the Baltic. Finally, various factors possibly contributing to the complex pattern of variation in the Swedish population ofButeo buteoare discussed.