The isonymy structure of trilingual Belgium was studied using the surname distributions for 1,118,004 private telephone users. The users were distributed in 77 Flemish, 76 French, and 3 German speaking towns, selected on a geographic basis to form an approximately regular grid over Belgium. Lasker's distance was found to be considerably higher between languages than within languages. For the whole of Belgium, irrespective of language, it was highly correlated with linear geographic distance, with r = 0.721±0.014, which is the highest correlation observed in European countries to date. Within Belgium and within languages, the correlation was highest among the Flemish (r = 0.878 ± 0.007), and lowest among the French (r = 0.631±0.020). Isolation by distance in Belgium is the highest we have found in Europe, and as high as in Switzerland where the different languages are separated by geographical barriers. This is not the case in Belgium, so that the considerable isolating power of languages emerges clearly from the present analysis. From the comparison of Lasker's distance between (9.48) and within (8.16) languages, and from its regression over geographic distance (b = 0.01206), it was possible to establish a quantitative relationship between the isolating power of languages and that of geographic distance as (9.48–8.16)/0.01206 = 109 kilometres. This transformation of language distance into an equivalent geographic distance, given here for Belgium, can be applied to any similar geo-linguistic situation.