The lysogenic state of the methylotrophic strain Acetobacter MB 58/1 is completely demonstrated by curing and lysogenization experiments. During these investigations we found that some phenotypic characteristics are modified by the presence or loss of the prophage MO 1. It could be shown that changes of the serological behaviour, the adsorption of the phages and the sensitivity against oxytetracycline are caused by lysogenic conversion. The phenotypic alterations of the bacterial cells induced by the phage genome are the result of modifications of the lipopolysaccharide structures on the cell surface. In the case of oxytetracycline resistance, interactions between the modified lipopolysaccharide structures and specific transport proteins of the cell membranes must be assumed, Durch Induktion der lytischen Entwicklung des Prophagen MO 1 nach UV-Bestrahlung wurde der lysogene Zustand des fakultativ methanolassimilierenden Acetobacter-Stammes MB 58/1 nachgewiesen. Aus einer Population von Acetobacter MB 58 konnte der fur den Phagen MO 1 sensible Indikatorstamm Acetobacter MB 58/4 isoliert werden. Lysogener Stamm und Indikatorstamm stimmten in den zur taxonomischen Einordnung zu verwendenden Merkmalen iiberein, unterschieden sich erwartungsgemiiB eindeutig in ihrer Reaktion gegen den Phagen MO 1 (WUNSCHE et al. 1983