The peroxidase activity in a single protoplast of alga Bryopsis plumosa is quantitatively characterized by scanning electrochemical microscopy. The generation of ferriceniummethanol (FMA+) at the protoplast surface is directly detected by the microelectrode tip scanned close to the sample surface in seawater containing ferrocenemethanol (FMA) and hydrogen peroxide, an electron mediator and an enzyme-substrate, respectively. The oxidation reaction requires hydrogen peroxide, which clearly shows FMA+ generation due to the peroxidase (POD) catalytic reaction occurring in the protoplast. The FMA+ generation and the FMA accumulation rates at a single alga protoplast were equivalent. A plot of the FMA+ generation rates according to the hydrogen peroxide concentrations was well allowed with a Michaelis-Menten- type reaction. An estimation of the mass-transfer rate and a determination of the Km are quite important advantages of the SECM technique that cannot be realized using other techniques. The POD activity has been further investigated from the viewpoint of the size of the protoplast. The POD activity of the alga in the adult stage is also visualized by SECM. The noninvasive nature of the SECM technique has been confirmed by observing the developmental process after measurements.