The current paper is devoted to the investigation of ascidian circulating hemocytes populations which are used as an example of blood cell specialization in the lower Chordates. Work has been performed on a solitary ascidian Halocynthis aurantium from the Japanese Sea. Using light microscopy and histiochemistry, we have identified five main populations of circulating hemocytes (hemoblasts, granulocytes, hyaline amoebocytes, macrophage-like and morula cells), which are typical of all tunicates. Pigment cells were not revealed. The possible pathway of circulating cells differentiation is assumed.