The O3 concentration in the marine boundary layer over the northern North Pacific Ocean was measured on a commercial vessel. No significant longitudinal gradient was seen over the area between 170¿E and 135¿W. A spring O3 maximum and summer minimum were observed. A diurnal cycle of O3 was found, with a high nighttime concentration and a low daytime concentration. Throughout the year an O3 decrease was clearly observed just after sunrise, which is known as sunrise ozone destruction (SOD). SOD may act as an important O3 sink in the marine boundary layer. Dramatic O3 destruction (2.5--3 ppb hour-1) just after sunrise was found in spring 2001. The cause of this phenomenon is discussed.