<p>The retrieval algorithm for the column mixing ratio of CO<sub>2</sub> from the measurements of a pulsed multi-wavelength integrated path differential absorption (IPDA) lidar is described. The lidar samples the shape of the 1572.33 nm CO<sub>2</sub> absorption line at 15 or 30 wavelengths. The algorithm uses a least-squares fit between the CO<sub>2</sub> line shape computed from a layered atmosphere model to that sampled by the lidar. In addition to the column average CO<sub>2</sub> dry air mole fraction (XCO<sub>2</sub>), several other parameters are also solved simultaneously from the fit. These include the Doppler shift in the received laser signal wavelengths, the product of the surface reflectivity and atmospheric transmission and a linear trend in the lidar receiver's spectral response. The algorithm can also be used to solve for the average water vapor mixing ratio, which causes a secondary absorption in the wings of the CO<sub>2</sub> absorption line under high humidity conditions. The least-squares fit is linearized about the expected XCO<sub>2</sub> value which allows the use of a standard linear least-squares fitting method and software tools. The standard deviation of the retrieved XCO<sub>2</sub> is obtained from covariance matrix of the fit. An averaging kernel is defined similarly to that used for passive trace-gas sounding. Examples are presented of using the algorithm to retrieve XCO<sub>2</sub> from the measurements from NASA Goddard's airborne CO<sub>2</sub> Sounder lidar made at a constant altitude and during spiral-down maneuvers.</p>