In this paper we study the manuscript tradition of Petrus Alfonsi’s Dialogus contra Iudaeos, written around AD 1110. This text was widely disseminated in the Middle Ages, especially during the century after its composition; there are over sixty complete manuscripts known. In order to
group them we calculate a distance matrix from standardised text strings transcribed from the manuscripts. From this, tree graphs can be generated easily and quickly with the aid of software developed for biological phylogeny. The resulting tree graph can be iteratively improved by modifying the distance matrix using a number of methods, partly fully algorithmic, partly relying on philological decisions. We are thus able to divide the tradition into some ten main groups.