The present mograph is the fourth of six volumes which review the Hypotricha, a major group of the spirotrichs. The book is about the Gostomatidae, the Kahliellidae, and some taxa of unkwn position in the hypotrichs. Gostomum was previously misclassified in the Oxytrichidae because its type species Gostomum affine has basically an 18-cirri pattern, which is dominant in the oxytrichids. A new hypothesis, considering also molecular data, postulates that this 18-cirri pattern evolved in the last common ancestor of the hypotrichs and therefore it appears throughout the Hypotricha tree. The simple dorsal kinety pattern, composed of only three bipolar dorsal kineties, and gene sequence analyses strongly suggest that Gostomum branches off rather early in the phylogenetic tree. Thus, the Gostomatidae, previously synymised with the oxytrichids, are reactivated to include the name-bearing type genus and other genera (e.g., Paragostomum, Wallackia, Cladotricha) which have the characteristic gostomatid oral apparatus. The Kahliellidae are a rather vague group mainly defined via the preservation of parts of the parental infraciliature. The kahliellids preliminary comprise, besides the name-bearing type genus Kahliella, genera such as Parakahliella and its African pendant Afrokahliella or the motypic Engelmanniella. In total 68 species distributed in 21 genera and subgenera are revised. As in the previous volumes almost all morphological, morphogenetic, molecular, faunistic, and ecological data, scattered in almost 700 papers, are compiled so that the four volumes (Oxytrichidae, Urostyloidea, Amphisiellidae and Trachelostylidae, Gostomatidae and Kahliellida) provide a detailed insight into the biology of almost 500 species of hypotrichs. The series is an up-to-date overview about this highly interesting taxon of spirotrichous ciliates mainly addressed to taxomists, cell biologists, ecologists, molecular biologists, and practitioners.