A sensitive PCR method amplifying an internal fragment of the major tail protein gene was developed to detect Lactobacillus delbrueckii subsp. lactis lytic bacteriophages in undefined, thermophilic whey starters used in Italy for production of Grana and Provolone cheeses. PCR was applied to several lytic Lb. delbrueckii subsp. lactis bacteriophages, which were highly diverse according to restriction analysis and phage host range. PCR detected the presence of phages in two out of 11 cultures, when applied to whey starters for Grana Padano cheese sampled from different cheese plants. The presence of actively growing phages in infected cultures was confirmed by traditional test. The PCR method proved to be useful to screen for the presence of Lb. delbrueckii subsp. lactis phages in thermophilic whey starters.