Most activated sludge treatment plants suffer from the presence of foams on the surfaces of their aeration reactors. These are often stabilized by hydrophobic mycolic acid-synthesizing actinobacterial species. A polyvalent Siphoviridae phage, GTE7, which lysed several Gordonia and Nocardia species, is described here. Its genome has a modular structure similar to that described for Rhodococcus phage ReqiDocB7. In laboratory-scale experiments, we showed that GTE7 prevents stabilization of foams by these Gordonia and Nocardia species.