Diatoms are the major contributors to phytoplankton blooms in lakes and in the sea and hence play a central role in aquatic ecosystems and the global carbon cycle1. All free-living diatoms differ from other phytoplankton groups in having silicified cell walls in the form of two shells (the frustule) of manifold shape and intricate architecture2 whose function and role, if any, in contributing to the evolutionary success of diatoms is under debate3, 4, 5. We explored the defence potential of the frustules as armour against predators by measuring their strength. Real and virtual loading tests (using calibrated glass microneedles and Finite Element Analysis) were performed on centric and pennate diatom cells. We show for the first time that the frustules are remarkably strong by virtue of their architecture and the material properties of the diatom silica. We conclude that diatom frustules have evolved as mechanical protection for the cells because exceptional force is required to break them. The evolutionary arms race between diatoms and their specialised predators will have had considerable influence in structuring pelagic food webs and biogeochemical cycles.