The thymine requirement of the E. coli strain HF 4704 (uvr A-, rec A+) is thermosensitive i.e. these cells require for their growth 2 μg thymine per ml at 37°C but not at 30°C. Such cells when starved for thymine for 3 h at 37°C are capable of sustaining growth of single stranded DNA phage φ{symbol}X174 without any diminution of burst size under nonpermissive conditions. Thymine starved HF 4704 cells also reactivate UV-irradiated φ{symbol}X174 by about 3fold. To test if the thymine necessary for phage growth under "thymineless" conditions was supplied by host DNA degradation products, the transfer of 32P label from the host DNA to mature progeny phages was measured by means of sucrose density gradient analysis. It was found that only about 0.7% of 32P of the host DNA was transferred to the progeny phages growing in normal cells whereas the corresponding value was 7.8% in the case of thymine starved cells. © 1977 Springer-Verlag.