Airborne observations of atmospheric methane (CH4) mixing ratio were carried out over Siberia during July 15--30, 1993, using a continuous measurement system. The distribution of CH4 in the upper troposphere at an altitude of about 7 km was highly variable (1.72--1.91 parts per million, or ppm), being affected by intrusion of stratospheric air, long-range transport of CH4 from the source region, and a synoptic-scale mixing process. During flights along nearly constant latitude between Yakutsk (62.10 ¿N, 129.50 ¿E) and Nizhnevartovsk (60.57 ¿N, 76.40 ¿E) on July 19 and 28 we observed large-scale enhancements of CH4 from near 90¿ to 100 ¿E with a horizontal scale of more than 500 km. Accompanied by high humidity and low mixing ratios of CO2 and O3, the air masses with enhanced CH4 mixing ratio were considered to be affected by lower tropospheric air. The probable source region of the CH4 was inferred to be the West Siberian Lowland, which contains vast wetlands and huge oil/gas fields. The distribution of CH4 in the lower troposphere (0.1--3 km) over the boreal wetlands in the West Siberian Lowland, ranging from 1.85 ppm to 2.75 ppm, showed large accumulations near the ground in the morning. The short-term temporal variations in the vertical profile of CH4 mixing ratio revealed that the accumulated CH4 was transported to the free troposphere with the growth of the mixed layer. On the other hand, the distribution of CH4 (0.1--1 km) over the tundra areas in the Siberian Arctic was less variable (1.81--1.84 ppm), and little accumulation was observed. ¿ 1997 American Geophysical Union