An experimental study of the two‐plasmon decay instability is reported. An intense CO2laser (10.6 &mgr;m) was used to generate the instability in a long scale length, preformed plasma. The principal experimental diagnostic measurement was time‐resolved Thomson scattering, with resolution in either &ohgr; orkspace. The results show that the instability evolves basically in agreement with linear theories, then saturates by the nonlinear process of mode coupling to short‐wavelength ion fluctuations. At saturation, density fluctuations close to 20% are observed in both the electron and ion waves.