Iron limitation often results in increased cellular silica contents of diatoms, suggesting that diatoms grow thicker and possibly mechanically stronger frustules when limited. We performed stability measurements for six diatom species grown under iron-limitation and iron-sufficient conditions. Frustule strength increased in all species when grown under iron limitation, with this effect being statistically significant for four of them. Valve morphology and silica content of the pennate Fragilariopsis kerguelensis and the centric Coscinodiscus wailesii changed under iron limitation but only valve morphology changes were significant; F. kerguelensis grew thicker costae while C. wailesii had smaller pores, especially in the outer part of the valves. These morphological changes are clearly in agreement with increased mechanical strength. Increased cellular silica concentrations in diatoms grown under iron limitation do result in increased frustule strength, most likely improving their protection against grazers.