We have measured the Rayleigh linewidth in CO2 in the critical region using a self-beat spectrometer. The linewidth was measured as a function of both temperature and cell height. The thermal diffusivity χ calculated using the Landau-Placzek equation is in excellent agreement with the values that have been obtained by thermodynamic measurements at three temperatures within the temperature range we investigated (T-Tc=-1.04, +1.06, and +3.8C°). Thus the Landau-Placzek equation is directly verified in the critical region, at least for temperatures not too close to Tc. However, we find that very near Tc [for ∊≡(T-Tc)Tc<~10-4], the correlation length in CO2 is of sufficiently long range (~250 Å at ∊=10-4) to require that the Fixman-modified linewidth equation be used in order to correctly describe the linewidth behavior. The thermal diffusivity was obtained along the critical isochore for the temperature range 0.02<=(T-Tc)<=5.3C∘ and along both the gas and liquid sides of the coexistence line for 0.02<=(Tc-T)<=2.3 C°. The results are (in units of cm2/sec): along the critical isochore, χ=(18.1+/-0.5)×10-6(T- Tc)0.73+/-0.02 along the gas coexistence line, χ=(36.0+/-3.0)×10-6(Tc- T)0.66+/-0.05 and along the liquid coexistence line, χ=(34.8+/-2.5)×10-6(Tc- T)0.72+/-0.05. These exponents are in reasonable quantitative agreement with the prediction of Kadanoff and Swift that χ~|∊|-ν(ν~23). Our exponents are also in accord with the thermal-conductivity divergence λ~∊-12 predicted by Fixman and by Mountain and Zwanzig, if the isothermal compressibility diverges as ∊-54, as predicted by the Ising model. Thus both theory and experiment indicate a stronger divergence in the thermal conductivity than has heretofore been assumed. Our subcritical exponents are also in agreement with the linewidth measurements by Saxman and Benedek in SF6; however, above the critical temperature they obtained an exponent of 1.27, in definite disagreement with our result.