The general organization of arthropod epithelia is compared to that of vertebrates. It is suggested that although ciliated epithelia, stratified epithelia and in some cases continuous muscle sheaths do not occur in arthropods, they have certain analogous structures which carry out the same functions. For example, the arthropod cuticle is compared to the squamous layer of vertebrate stratified epithelia, and complex arthropod basement membranes are compared to the muscle and connective tissue sheaths of certain vertebrate epithelia. The cellular organization of transporting epithelial cells is then discussed, with particular reference to elaboration of plasma membranes, and similarities and differences between vertebrates and arthropods, and between insects and crustaceans are pointed out. Specializations peculiar to insect cells are described, including the insertion of mitochondria into apical membrane microvilli, and the presence along this membrane of small particles called portasomes believed to be involved in active transport. Finally, it is shown that in the midgut of theinsect Manduca sexta, distinct ultrastructural changes accompany loss of potassium transport activity during a larval molt and in the prepupal stage. The ultrastructural changes which occur include a proliferation of the basement membrane and muscle tissue underlying the epithelium, and a change in the morphology of the potassium transporting goblet cells. Possible correlations between ultrastructural changes and loss of transport activity are discussed.