A considerable number of the thirteenth and early fourteenth-century manuscripts of Michael Scot's Arabic-Latin translation of Aristotle's De animalibus (ca. 1215) display a system of guiding marginal glosses. These glosses are usually added by a later hand with respect to the hand that had written the text. The manuscripts were not only annotated for personal use, but also so as to allow for a better use in compiling commentaries, encyclopaedias and compendia. We can say that the marginalia form the main, if not only, key to our understanding today of the use that readers made of the text. Apart from offering a mere explanation of the contents, we can see that their interest in the text was chiefly related to biblical exegesis and to scientific and medical knowledge. The approach to the text was often allegorical and moralizing in character. Some remarks reflect the reader's own experience and associations in relation to the text.