This study analyses one of the few musical testimonies of Hispanic origin related to the practice of Ars Subtilior. The fragment in question, preserved in residual pages of a liturgical codice (M1361), consists of a polyphonic amplifi cation of the Amen of a monosodium Crew. The manuscript is examined from different angles by clarifying issues such as the origin of the codice, the identity of its main writer and shedding new light on the subtilior fragment. The analysis of his writing process suggests a connection between the subtilious bogue and the practice of improvising biscanti on liturgical tenors. The author also reviews other imprecisely classified hispanic sources in musical literature such as Ars subtilior because of erroneous transcripts.