The forbidden d3Δi–X1Σ+ transition of CO has been observed in absorption at high resolution in the vacuum ultraviolet region. The intensity distribution in the rotational structure of the observed bands is in conformity with the assumption that the transition occurs on account of the interaction between the d3Δ state and a 1Π state, presumably the A1Π state. Thirteen bands of the d–X system have been analyzed yielding more extensive rotational data for the d3Δi state than were previously known. A discussion of the local perturbations in the d state by the A1Π and a3Π states is included.