The nucleoside antibiotic nikkomycin inhibited the formation of β‐chitin spines by the diatom Thalassiosira fluviatilis, and slowed its growth rate, at a minimum inhibitory concentration of 4 μM. The resulting chitin‐free cells sedimented much more rapidly in the cultures than did control cells. The uptake of nikkomycin was not antagonised by the dipeptides Gly‐Val, Met‐Met or Phe‐Leu. Of Six dipeptides tested, Met‐Met, Phe‐Leu and Tyr‐Gly were taken up by the diatoms, but Gly‐Gly, Gly‐Val and Ala‐Ala were not.