Zoospores at various developmental stages in Hydrodictyon reticulatum were isolated from parent cells and cultured in Waris medium. Isolated zoospores grew to mature vegetative cells, and were able to reproduce zoo‐spores that formed daughter hexagonal nets. Three types of shape appeared in cells 24 h after isolation: cylindrical, Y‐shaped and 4‐armed type. Protrusions of Y‐shaped or 4‐armed cells were formed at an angle of about 120° to the long axis of the cell. When cells were isolated at later stages, more cells became cylindrical in shape and fewer ceils became Y‐shaped or 4‐armed, Direction of cell growth also seemed to depend largely on the developmental stages of the zoospores. The later the isolated stages were, the more the cells elongated along the long axis of the zoospores. Copyright © 1995, Wiley Blackwell. All rights reserved