A study of the mycelium and haustoria of certain rusts of Abies has been made. It was found that the mycelium of three rusts have distinguishing characteristics. The haustoria of Melampsorella Cerastii, Hyalopsora Polypodii and Milesia polypodophila are so distinctly characteristic as to serve as diagnostic criteria for the species. The haustoria of six Uredinopsis species are similar in type; those of two species of Pucciniastrum are of the same type. The haustoria of Calyptospora Goeppertiana are hyphal and very slender. The relation of the haustoria of the rusts, up to near maturity, is one of high symbiotic nature in Abies balsamea. The haustoria of Milesia polypodophila and certain other rusts in their aecial hosts show a contrast in type to the haustoria in their telial hosts. The haustoria of Milesia Scolopendrii and M. vogesiaca are hyphal or of loose spiral form. The haustoria of M. Kriegeriana resemble those of M. intermedia. The haustoria of M. Polypodii are usually characteristically branched, serving as an aid in identification. Haustoria were found to occur in the leaves of 19 species of the Pucciniastreae on Abies. Eighteen of these are described in the aecial host for the first time.