Foaming in activated sludge plants is a worldwide problem commonly caused by proliferation of bacteria of the order Corynebacteriales. These include Skermania piniformis, a filamentous bacterium that has been documented to be a major cause of foaming globally, and particularly in Australian treatment plants. Phage SPI1 is the first phage that was isolated and shown to infect this organism. It targets seven of the nine strains of S. piniformis held in our culture collection, but none of the other 73 mycolata strains of different genera, mostly isolated from wastewater, against which it was tested. Phage SPI1 is a member of the family Siphoviridae and has a circularly permuted dsDNA genome of 55,748 bp with a G+C content of 67.8 mol %. It appears to be obligatorily lytic, with no evidence of genes related to a lysogenic mode of existence.