The infection of E. coli cells with different lambdoïd prophages triggers a stimulation of galactokinase synthesis when cells are grown in a medium giving rise to a mild catabolite repression (tryptone broth) with an inducer of the gal operon (fucose). These results show that during phage infection (or induction) some factor acting in trans is produced which is able to overcome efficiently catabolite repression of the kinase cistron. Using different strains of lambdapbio252 (pam, qam, "hl), lambdapbio256Hl and lambdaNNS7 we have concluded that the factor is the N gene product which is known for its anti- p(rho) action. Studies of the whole gal operon in the same conditions show that epimerase unlike transferase and galactokinase is practically insensitive to catabolite repression by tryptone broth and that viral development has a low effect on it. This indicates that there is an internal modulation of gal operon expression. A mRNA termination site sensitive to the p factor is known in the gal operon between galE and galT. Another site weaker than this one might exist between galE and operator-promoter region.