The scurfy scales in the genus Chionaspis comprise a unique taxon among the armored scale insect genera in North America. The taxonomic reviews of the species belonging to the genus by Cooley (1899) and Ferris (1937, 1942) are far outdated. The present research is a comprehensive review of all the species in this genus in North America, with special emphasis on polymorphism associated with feeding sites which has been discovered in some species. Seventeen species have been redescribed and illustrated in detail, each with a discussion of their morphological affinities and relationships. Their host habits and zoogeography are summarized. The five species having typical bark and leaf forms, as well as intermediate forms, are discussed in more detail. A separate chapter deals with polymorphism, including a historical literature review of the subject. A hypothesis is given on the modes of transfer from the bark form to the leaf form and vice versa, that results in different morphs in first or second generations. Two keys were prepared: one for the genera related to Chionaspis and another for the determination of the species of this genus in North America. The three species that have been recently redescribed, C. americana and C. kosztarabi and C. nyssae are also discussed, and the most important morphological characters and the plates prepared by the original authors for each of these three species are given in order to better utilize the key to the species in North America. As a direct result of this research, two new species have been discovered: Chionaspis gilli Liu and Kosztarab, and C. hamoni Liu and Kosztarab. In addition new morphs were found for C. platani and C. wistariae. This thesis also includes many new distribution and host records for several species. The phylogenetic relationship of all the species in this study has been discussed, and as a result of the Ward's Minimum Variance Cluster Analysis (1985 version, SAS), a dendrogram has been provided based on 23 numerical characters. / Master of Science