Lactococcus lactis ssp. cremoris C3 was shown to contain two prophages, C3-T1 and C3-T2, both inducible bymitomycin C. The phages were morphologically indistinguishable, having an isometric head (55 nm), a noncontractile tail (142 nm), and a distinctive base plate. The phages C3-T1 and C3-T2 were differentiated from one another by restriction endonuclease analysis of their DNA, and genome sizes of 37.8 (C3-T1) and 32.5 kb (C3-T2). Southern blot DNA hybridization suggested a maximum homology between the phage genomes of 50%. Phage C3-T1 was propagated and the phage obtained was designated C3-TIℓ. Phage C3-T2 was not propagated despite tests with a large number of possible indicator strains. Restriction enzyme analysis of phage C3-T1ℓ genome yielded a 37.8-kb circular restriction map. The packaging site (pac) and site of integration into the bacterial chromosome (att) were located and an EcoRI DNA fragment containing the att site was cloned. Only one C3-T1 att site was present in the C3 chromosome. No homology was detected between DNA from C3-T1ℓ and DNA from lytic phages commonly isolated from cheese wheys. Key words: lactococcal phages, temperate phages, lysogeny, evolution.