AbstractPhycocyanin‐free photosynthetic lamellae (PSI‐particles) were prepared fromAnacystis nidulans, grown in complete and iron‐deficient media. French press treatment and fractionated centrifugation were used. Absorption studies of the particles revealed an iron deficiency‐induced shift of the main red chlorophyllaabsorption peak from 679 to 673 nm as reported before for whole cells. The shift may reflect a changed distribution between different chlorophyllaforms. Action spectra for photo‐oxidation of mammalian cytochromecwith photosynthetic lamellae revealed an iron deficiency‐induced shift, corresponding to that found in the absorption spectra. As photo‐oxidation of cytochromecis mediated by PSI, it is believed that chlorophyllaalso after the shift towards shorter wavelengths, is active in PSI.A decreased photosynthetic capacity of PSI, due to iron deficiency, was shown by time course studies of photosynthetic oxygen evolution, by photo‐oxidation studies of P700 and mammalian cytochromec, by photo‐reduction studies of NADP and by combined studies of light‐induced and chemical oxidation of P700. The ration chlorophylla/700 was also determined for whole cells, lyophilized cells and PSI‐particles. Iron deficiency caused an increased ratio in all studied fractions. The results of this work imply that energy is transferred with less efficiency within the photosynthetic units of PSI in iron‐deficient A. nidulans than