The rhabdomeres of cephalopod photoreceptors, which are built up mainly of rhodopsin and phospholipid molecules, show a very high alkaline phosphatase activity. The enzyme has been partially characterized in purified rhodopsin vesicle fractions of the rhabdomeres by the following kinetic data: pH optimum 8.7; activation energy 9100 cal . m-1; Vmax = 2.5 mumol . min-1 . mg-1; Km = 1.5 x 10(-4) M; its activity depends on Mg2+. There is good evidence that the alkaline phosphatase is a membrane-bound enzyme with receptor sites presumably located on the inside of the membrane. This enzyme has not been purified but its high activity compared to that of other known alkaline phosphatases (see Table I) indicates that each mirovillus, the structural unit of the rhabdomere, contains 1-20 enzyme molecules. This finding supports the hypothesis that the alkaline phosphatase is involved in the biochemical amplification process of excitation, or adaptation.