The defective phages ofBacillus subtilis cannot be counted by plating as they do not form plaques. In addition, counting under the electron microscope with latex spheres as an internal standard is not possible. The reliability of a method usingEscherichia coli phage T4 as a substitute for the latex spheres has been tested and the results compared with those of other methods. Using this method, we determined the burst sizes of the defective phages PBS X, PBS Y and PBS Z under various conditions.