The publication draws on philological, translatological, and language-historical methodologies to investigate an Old Spanish manuscript of Aristotle's »De animalibus«. It also provides an appreciation of the (translated) work's significance for the history of science. In the study, extracts from the manuscript are edited, compared with the Arabic and Latin text, and commented upon for the first time. The outcome of this close examination is that two translators were involved, probably in the second half of the 13th century, and that they translated simultaneously from Arabic and Latin into Castilian, an approach not otherwise evidenced so far.