We report on the use of degenerate four-wave mixing in a forward beam configuration for the detection of C2 in a premixed acetylene-oxygen flame. The Swan system, d3Πg - a3Πu (0,0), near the P branch bandhead of the C2 molecules was measured. The spectrum shows clearly resolved rotational structures of the P and R triplets. For diagnostic applications such a spectrum gives information on the molecular concentration as well as on the temperature in the probed volume. The rotational temperature of the C2 radical in the measurement point was determined from the experimental data.