Colony formation is common feature among non-motile marine phytoplankton. Several theories exist around the potential benefits of larger colonies. Here we test the hypothesis that predation is one of the drivers behind colony formation and chain length plasticity. We exposed cultures of Thalassiosira rotula, Chaetoceros curvisetus and Chaetoceros affinis to copepodamides, a chemical alarm cue released by copepods and perceived as an indicator of predation threat by their prey. This was coupled with a grazing experiment which compared copepod grazing rates on different chain lengths. Our results show that T. rotula and C. curvisetus decreased their chain lengths by 79% and 49%, respectively, in response to copepodamides. Single cells and short chains were grazed at lower rates compared to long chains and the copepodamide driven size shift led to 30% and 12% lower grazing in T. rotula and C. curvisetus respectively. In contrast, C. affinis showed a slight increased chain length in response to copepodamides although non-significant. We found that 2 of 3 studied species reduce their chain length in response to the presence copepod grazers. Altered size structure has implications for the route of carbon in the marine food webs and carbon export to deeper strata.