Diatoms are encased within sophisticated stable lightweight silica cell walls. These frustules have the potential to protect the algal cell against the feeding tools of their most abundant metazoan predators, the copepods. We examined the mechanical strengths of the 3 North Sea diatom species Actinoptychus senarius, Thalassiosira punctigera and Coscinodiscus wailesii and their effect on feeding efficiency of copepods. (1) We determined the stability of the diatoms by means of ‘micro-crush-tests’ performed in the laboratory with calibrated microneedles. (2) In feeding experiments, we compared the ability and efficiency of the 3 North Sea copepod species Temora longicornis, Centropages hamatus and Acartia clausi to crush frustules. The results showed a remarkable correlation between mechanical properties and size of diatom frustules and feeding success of the copepods. The weakly silicified diatom T. punctigera was the least stable and best fed upon, whilst having the highest growth rate. The diatoms having the most complex frustule, A. senarius, exhibited the greatest stability, whilst being fed upon least. The largest diatom, C. wailesii, was partially protected by its size, but was nonetheless suitable as prey for the large copepods that, in the case of C. hamatus, seem to have developed special feeding techniques to overcome the size-mediated protection.