The nd sigma and nd delta (n=4...9, upsilon=0) triplet Rydberg series of carbon monoxide have been observed for the first time and rotationally resolved with an accuracy of 0.03 cm(-1) using a three-step excitation scheme. A multichannel quantum defect theory (MQDT) analysis shows strong perturbations for n=5, 8, and 9 and predicts the positions of the s sigma and d pi levels. There is significant mixing between the s sigma and d sigma states, with a mixing angle between 42 and 45 degrees, as well as some smaller interaction with the np series. The ionization limit for CO is found to be 113 027.5(3) cm(-1).