The study is divided into two parts. The first dealing with the complex rhizophlyctoid alliance (Rhizophlyctis-Karlingia-Karlingiomyces) and the second, with chytrids inhabiting insect exuviae and belonging in the genera Siphonaria, Diplophlyctis (Asterophlyctis), and Rhizoclosmatium. The organisms studied were brought into axenic or unichytrid culture. A detailed account of the isolation procedure is given. The developmental, morphologic, and taxonomic aspects of the following species are discussed and all are illustrated: Rhizophlyctis aurantiaca n. sp., R. reynoldsii n. sp., R. serpentinus n. sp., R. variabilis Karling var. burmaensis var. nov., Rhizophlyctis sp. D-1, Karlingia granulata Karling, Karlingiomyces dubius (Karling) Sparrow, Siphonaria variabilis H. E. Petersen emend., Diplophlyctis (Asterophlyctis) sarcoptoides (H. E. Petersen) comb. nov., D. asteroidea n. sp., D. versiformis n. sp., ?Rhizoclosmatium sp., Septochytrium willoughbyii n. sp., and Chytridium schenkii (Schenk) Scherffel var. dumontii var. nov. Data on the germination of resting spores in Rhizophydium spinosum Willoughby is also given. Ninety-three monosporic isolates of the ubiquitous and controversial "Karlingia rosea" were studied comparatively. These could not be distinguished from one another on a morphological basis. Resting spores are absent in 63 isolates and present in 30. Of the latter, 23 form them asexually and the rest, sexually. Preliminary experiments on pairing of isolates not forming resting spores failed to verify the alleged occurrence of "heterothallism". Dormancy and rejuvenation were studied in detail. The factors involved were thought to have a bearing on the cosmopolitan distribution of "K. rosea". The systematics of the latter are discussed.