Computational dialectometry is a multidisciplinary field that uses quantitative methods in order to measure linguistic differences between the dialects. The distances between the dialects are measured at different levels (phonetic, lexical, syntactic) by aggregating over the entire data set. These aggregate analyses do not expose the underlying linguistic structure, i.e. the specific linguistic elements that contributed to the differences between the dialects. The aim of the work presented here is to extract the linguistic structure from the aggregate analysis of Bulgarian dialects carried out on the basis of word pronunciations. The method is based on the identification of regular sound correspondences and is for the first time applied in dialectometry in order to extract linguistic structure from the aggregate analysis. All the analyses are based on the transcriptions of 117 words collected from 84 sites equally distributed over the Bulgarian dialect area. The results have shown that identification of regular sound correspondences can be successfully applied in the task of identifying linguistic structure in the aggregate analysis of dialects based on word pronunciation.