For production of cyanobacterial gas vesicles, polyethyleneimine (PEI) was used as flocculent to enhance the collection of buoyant Anabaena flos-aquae cells by flotation. During the flocculation-flotation process, PEI was found to cause the cells to release blue pigment(s) (presumably phycocyanin) and probably other cytoplasmic materials. The kinetic profile of the pigment release was followed and compared with that of the culture flotation. The effects of PEI on the cyanobacterial cells were also examined using transmission electron microscopy. Unlike the previous reports for the effects of PEI on Gram-negative bacteria, our observations did not reveal significant morphological changes of the PEI-exposed cells of A. flos-aquae. There were no apparent negative effects on the desired product, gas vesicles, during the PEI-induced flocculation-flotation process for cell collection.