We examined the effects of the intensity of incident light and the concentrations of cyanobacterium, Synechococcus sp. UTEX2380, and an electron transfer mediator, 2-hydroxy-1,4-naphthoquinone (HNQ), on the current output of a photosynthetic electrochemical cell operated under illumination. Though the current density was higher with increasing concentration of Synechococcus sp., increase in the current density was suppressed at more than 24 mu g chlorophyll/ml. The current density reached a maximum value at 1 mM HNQ. The current density was saturated at 50 W/m(2) probably due to reoxidation of HNQ by oxygen photosynthetically evolved. The conversion efficiency of light energy to electrical energy was 3.3%. For the analysis of energy loss for four steps in the electrochemical cell reaction, the energy loss remarkably arose from the reaction of the electron transfer chain and the high internal resistance of the electrochemical cell.