We report a series of sub-Doppler frequency measurements on four isotopic species of carbon monoxide (CO). The observed rovibrational transitions in the electronic ground stateX1Σ+of CO cover the infrared spectral region between 58.5 and 63.3 THz (1951–2112 cm−1). The CO laser heterodyne spectrometer with optional microwave sideband generation utilizes heterodyne techniques to measure the frequency of a CO laser, which is locked to saturation signals of CO transitions at low pressure, against combination frequencies of two saturation-stabilized CO2lasers. Frequencies for ground state transitions of three isotopic species of CO are determined with very high accuracy, including the first heterodyne frequency measurements in the fundamental band of13C18O. A systematic correction (between −4 and −13 kHz) of our earlier published CO sub-Doppler frequencies became possible with the 1994 improvement of the CO2laser standard frequency values. The updated frequencies together with the new measurements exhibit absolute frequency uncertainties around 20 kHz (Δν/ν ≈ 3 × 10−10). They are the most accurate frequencies of CO ground state transitions suitable for secondary frequency standards and calibration of infrared spectrometers in the 60-THz spectral region.