To develop a practical countermeasure for eliminating harmful algal blooms (HABs), microbiological algicidity has recently been highlighted. In 1996, a virus (HaV) specifically infectious to a representative harmful bloom causing microalga, Heterosigma akashiwo, was isolated from the natural seawater. The virus has at least 4 characteristics required for biological agents to prevent H. akashiwo blooms in the natural environment: (1) HaV was isolated from “natural seawater” and experienced no artificial operation for its DNA, (2) HaV attacks H. akashiwo quite specifically, (3) HaV reproduces itself only by infecting to the host, H. akashiwo, (4) production of a small amount of HaV at a moderate price has been succeeded.