A 2 + 1 double-resonance laser-induced fluorescence technique is described in which the high power of a pulsed laser is combined with the narrow bandwidth of a single-mode cw ring dye laser. The combination of these laser systems allows the study of highly excited states with MHz resolution. This double-resonance technique has been applied to the study of the B-1 Sigma(+)(upsilon' = 0) <-- A1 Pi(upsilon'' = 0) system of CO. From the high resolution spectra the term value of the A state could be determined, T = 64748.0091 +/- 0.0019 cm(-1), corresponding to an increased accuracy over previously reported values by an order of magnitude.