Tropospheric BrO profiles (about 0.6¿0.2 ppt, and 2.0¿0.8 ppt at profile maximum) were measured for the first time. Our measurements add new information to recent speculations-based on indirect evidence-of BrO possibly being ubiquitous in the free troposphere [Harder et al., 1998; Frie¿ et al., 1999; Van Roozendael et al., 1999; Pundt et al., 2000>. Our study relies on a detailed comparison of BrO slant column densities (BrO-SCD) measured in the troposphere from the LPMA/DOAS (Laboratoire de Physique Mol¿culaire et Applications and Differential Optical Absorption Spectroscopy) gondola by direct Sun absorption, and BrO-SCD values subsequently measured in the lowermost stratosphere during balloon ascent. The difference in total atmospheric BrO-SCDs measured in the troposphere and lowermost stratosphere-after a suitable correction for the change in BrO due to photochemistry and the observation geometry-is then attributed to tropospheric BrO. ¿ 2000 American Geophysical Union