Accordingt o a currenth ypothesis,t he colourp olymorphismin populationso f the adder,Vipera berus L., is maintained by a thermal superiority of melanistic snakes,which enables them to grow more quickly than normally coloured ones. Since largermales are superior in sexual combats, and larger females get more offspring, thisclearly should favour the melanistic trait. On the other hand, melanistic individualsare believed to suffer a higher predation pressure due to their more conspicuous appearance.T he predictionf rom this hypothesisi s that melanistici ndividualso n averageshould be largert han normali ndividualsi n mixed populations.T his predictionwas tested on adders captured on several small islands in the Stockholm archipelago(N 59°20';E 19°20').N o significantd ifferencew as found in weight, lengtho r weight/length ratio between melanistic and normally coloured male adders. Neither slopenor elevation of the regression lines of length on weight differed. Thus, the hypothesiswas not supported by our data.