Indirect immunoferritin labeling provided evidence for the de novo synthesis of antigenic cell-surface components of the daughter cell (bud) of a freshwater isolate belonging to morphotype IV of theBlastocaulis-Planctomyces group of budding and nonprosthecately appendaged prokaryotes. The cell surfaces of the buds produced by immunoferritin-labeled mother cells remained unlabeled through development to flagellated swarmers. Double-labeling experiments verified the de novo synthesis of cell-surface antigens over the entire bud, and also showed a labeling pattern indicative of intercalation of newly formed sites on the surface of the mother cell during the later stages of its maturation.