Diode-laser absorption spectra of a3Π CO between 1635 and 1780 cm-1have been measured in an a.c. discharge using population modulation detection, and assigned to vibration-rotation transitions of the fundamental and 1–2, 2–3, 3–4 hot bands in the Θ = 0, 1 and 2 fine structure states. The analysis was based on theR2model hamiltonian of Effantinet al.(1982) and thev= 0-4 data were fitted simultaneously with radio-frequency and microwave measurements, and with the perturbation by the a′3Σ+(v= 0) level included explicitly.