Two late 19th century catalogues of the collection of Mexican codices at the Bibliothéque Nationale, Paris, identify the support of all the manuscripts written on bark as genus Agave. Staining and microscopic examination of fibers from 22 codices on beaten bark showed that the fibers of the support were in fact from plants of genus Castilloa or Ficus. Earlier research has established that the indigenous form of paper using these two fibers originated in hot regions of Mexico. Leaves of the genus Agave were used in cold regions and fibers of the genus Yucca in temperate regions. Fiber identification may establish the origin of manuscripts. These indigenous papers were used in both preand post-Hispanic manuscripts. Other supports of pre-Hispanic codices include cotton fabric and stag or jaguar hide. Some post-Hispanic codices use European papers. In two of these, linen fibers were microscopically identified.