We have conducted GPS radiosonde and ozonesonde observations on board the research vessel “Shoyo-Maru” in the equatorial eastern Pacific. These observations took place in September and October 1999 as a part of the Soundings of Ozone and Water in the Equatorial Region (SOWER)/Pacific mission. The mean profile of ozone is similar to that for the dry season (September to October) at San Cristóbal, Galápagos (0.9°S, 89.6°W) located in the equatorial eastern Pacific. The mean tropospheric ozone concentration is about 40 ppbv with a maximum in the mid-troposphere. Compared with the mean profile during the dry season at Watukosek, Indonesia (7.5°S, 112.6°E), this mid-tropospheric maximum is larger, and a sharp increase of ozone below the tropopause begins at a lower altitude for the Shoyo-Maru profile. We frequently observed layers in which ozone and humidity are highly anti-correlated. These layers have vertical scales from several kilometers to several hundred meters. Horizontal scales of these layers are roughly 1000 km, which may correspond to time scales of about 2 days, since the vessel sailed about 500 km/day. These layers are related to northerly winds, which bring in wet and ozone-poor air from the inter-tropical convergence zone situated in the northern side of the main cruise track. Similar layers were observed in ozone profiles at San Cristóbal and Watukosek, mostly during the dry season, suggesting the existence of layers advected without vertical mixing.