A high phosphorus concentration is widely accepted as favorable for enhancing both microalgae growth and lipid accumulation; however, excessively high P could be counter-productive. In this study, we investigated the effects of increasing P levels (5.4, 25, 45, 150, and 250 mg-P L-1) on the heterotrophic cultivation of Chlorella regularis. Microalgae growth was inhibited and cells were severely damaged in response to highly excessive P levels (>= 150 mg-P L-1). In particular, 250 mg-P L-1 resulted in a similar to 40% decrease in cell density and a similar to 70% loss of cell viability. Microalgae damage induced by excessive phosphorus included enlarged cell size, deformation of cell walls, and disorganization of organelles. These negative effects were associated with the over-accumulation of polyphosphates within cells, which may further cause binding of P to intracellular components. Although P is an essential nutrient, excessive P lowers cell growth and viability.