Streptococcus thermophilus CNRZ1205 is the lysogenic host for the temperate phage oO1205. A derivative of CNRZ1205 was isolated which was cured of oO1205 and this strain was used to construct a re-lysogenised derivative. Pulse field gel electrophoresis and sequencing of the attachment site regions confirmed that excision and re-integration of the phage was a site-specific event. Interestingly, cells from the cured, as well as its re-lysogenised derivative, were found to have a very long chain length. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.