An instrument on the Pathfinder lander has been designed to measure the structure of Mars' atmosphere during spacecraft entry and descent from ~150 km altitude to the surface, and to measure meteorological parameters after landing for the landed duration of the mission. This is specified to be nominally 30 Mars days but potentially up to 1 Earth year. Landed sensors will measure surface level pressure; temperatures at 0.25, 0.5, and 1.0 m above the surface; and wind speed and direction at a height of 1.1 m. These sensors are mounted on a slender mast about 1.63 m from the center of the Lander base petal so as to avoid flow disturbance and thermal contamination insofar as possible. Wind sensing is most sensitive at velocities <20 m s-1 but can resolve speeds up to 50 m s-1, with directional accuracy ~10¿. A key point of interest in atmosphere structure is comparison with profiles obtained by Viking 20 years ago, to evaluate changes predicted to occur with solar activity and with dust loading of the atmosphere. The proximity of the landing site to that of Viking Lander 1 will permit comparison with and extension of the earlier lander data, which were taken over a period of more than 3 Mars years. The improved sensitivity of the lander instruments will also permit investigation of many new phenomena.¿ 1997 American Geophysical Union