During the Amazon Boundary Layer Experiment (ABLE 2A), approximately 100 hours of in situ, carbon monoxide (CO) measurements were obtained using a tunable diode laser sensor that provided continuous, fast-response (≈1 s) measurements. This extensive data set, taken on board the NASA Electra between July 12 and August 12, 1985, characterizes the temporal and spatial changes in the CO distribution over the Amazon basin as meteorological conditions progressed from the early to middle phases of the 1985 day season. Early dry-season CO values of 75--95 parts per billion by volume (ppbv) were generally observed in the planetary boundary layer (PBL). As the dry season became well established, biomass burning and a persistent capping inversion at ≈3 km profoundly altered the CO distribution, with PBL values typically reaching 150--275 ppbv. Concentrations as high as 800 ppbv were measured within identifiable haze layers. An analysis of satellite imagery shows that biomass burning was occurring along the Amazon River system and in savannah areas located more than 1000 km south of the region sampled. ¿ American Geophysical Union 1988