During the past two decades, the genetic diversities of bacterial and fungal communities and their relationship with the inhabited environments were intensively studied with the development of molecular biological techniques. However, although bacteriophages are ubiquitous and more abundant than their hosts in biosphere, their diversity is still little known. In this paper, we targeted the major capsid genes (g23) of T4-type bacteriophages and reviewed the recent progress on their genetic diversity. The distribution of g23 of T4-type bacteriophages was distinctly different among natural environments of marines, lakes and paddy fields, and the majority of g23 were grouped into several novel clusters according to their obtained environments. In addition, several research tips and future research tendencies for the study of environmental g23 were also addressed.