Bacteriophages, often referred to as phages, are viruses that infect bacteria. The genetic material in bacteriophages can be either DNA or RNA. This diversity in genetic material is one of the characteristics that distinguish different types of phages. Some bacteriophages, like the T4 phage, have DNA as their genetic material, while others, such as the MS2 phage, have RNA. This variety allows bacteriophages to adapt to different hosts and environments. Therefore, the correct answer is: Either DNA or RNA.