Microcystis aeruginosa mainly occurs as single cells in laboratory cultures, but in the colonial morph under natural conditions. This phenomenon suggests that some factors may be responsible for the typical occurrence of colonies in lakes. To demonstrate this possible effect, lake water with abundant or few zooplankton was filtered through a 0.15 µm membrane filter, and used as test water. Microcystis aeruginosa was cultured in BG-11 medium, with or without test water. The results showed that filtered lake water with abundant zooplankton could induce colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the treatment of filtered lake water with few zooplankton were still strongly dominated by unicells and paired cells, and no colony was formed. In the treatment of filtered lake water with abundant zooplankton, some colonies of several, dozens, and sometimes even hundreds of M. aeruginosa cells were formed. The unicells reduced to 53.4±6.5% and the proportion of cells in colonies increased from 0 to 24.5±4.6% of the populations. The mean number of cells per unit of M. aeruginosa in the treatment of filtered lake water with abundant zooplankton was promoted significantly compared with those of the control and the treatment of filtrated lake water with few zooplankton. In addition, no significant difference in growth rate was detected between the control and the treatments, regardless to the filtered lake water (i.e. with abundant zooplankton or with few zooplankton). Thus, we concluded that colony formation in our experiment was probably not associated with the nutrition difference but with some dissolved substances probably released from abundant zooplankton in the natural lake water, which may be one of the environmental factors responsible for the colonial form in M. aeruginosa.