A quasi-steady, highly underexpanded free jet of argon seeded with nitric oxide (NO) was generated at the exit of a converging, axisymmetric nozzle supplied by a shock-tunnel reservoir at 4200 K and 3.0 atm. During each run of the facility, an isolated transition in theA2Σ ←X2∏ (0, 0) band of NO at ∼ 226 nm was pumped with a pulse of frequency-doubled dye laser light formed into a thin sheet and directed perpendicularly through the axis of the jet. The red-shifted components of the resulting fluorescence at 90° with respect to the laser were imaged onto an intensified, charge-coupled device array. A ratio of images obtained by exciting lines originating from two different rotational states could be used to infer the mean rotational temperature field. However, because of the extreme variations in temperature and density present in the free jet, no single pair of lines simultaneously provided adequate signal levels and temperature sensitivity over the flow's entire temperature range (i.e., ∼ 100–3100 K). Instead, a combination of images obtained with four different transitions was used. Excellent agreement was observed between multi-line temperature evaluations from single-shot and frame-averaged images and a numerical simulation of the flow performed by the method of characteristics.