<p>Morphological plasticity widely exists in diazotrophic cyanobacteria, which mediates the cyanobacterial bloom development. Planktonic copepods are important grazers on filamentous cyanobacteria and release waterborne cues functioning as predation alarms in aquatic ecosystems. Here, the growth and morphological responses of the heterocystous cyanobacterium <i>Dolichospermum flos-aquae</i> to the grazing risk posed by the copepod <i>Cyclops</i> spp. were studied, with the effects of cues from copepods that had fed on <i>D. flos-aquae</i> and not fed on <i>D. flos-aquae</i> further compared. Results showed that <i>D. flos-aquae</i> suppressed the growth of long filaments under copepod cues exposure, with the formation of heterocysts functioning dinitrogen fixation inhibited in <i>D. flos-aquae</i>. In addition, the heterocyst placement pattern changed, with the number of vegetative cells between heterocysts increased under copepod cues. By contrast, the presence of copepod cues did not affect the physiology of vegetative cells including cell size and photosynthetic performance. While the suppressed growth of long filaments may reduce the grazing risk due to the large-prey preference by copepods, it brings the cost of reduced population growth rate in <i>D. flos-aquae</i>. The present findings reveal the morphological plasticity of heterocystous cyanobacteria to freshwater copepod grazing, highlighting the importance of biotic interactions in cyanobacterial blooms.</p>