The anonymous five-voice Missa Cueur langoreulx, which survives uniquely in a manuscript emanating from the atelier of Petrus Alamire, is an early example of a mass based on a polyphonic work by Josquin Desprez. At its heart lies an ingenious suite of ever-changing fuga-canons that rework and develop the one on which Josquin structured his five-voice chanson Cueur langoreulx. Through analysis of the canons and critical scrutiny of the polyphony, we conclude that the Missa Cueur langoreulxmay be by the enigmatic Mathurin Forestier, an early sixteenth-century composer who lacks a secure biography, but who is known from his attributed works to have had a keen interest both in complex canons and in Josquin’s music, and whose masses feature in a number of Alamire manuscripts.