We report observations of a relativistic plasma wave excited by two copropagating laser beams with frequency difference close to the plasma frequency in a fully ionized hydrogen plasma. The laser beams were at 1.064 µm (Nd:YAG) and 1.053 µm (Nd:YLF) of 200 ps duration and focused to a ≈ 7× diffraction limit spot with peak irradiance of 2×1014 W cm−2. The plasma wave was detected by monitoring the forward-scattered spectrum of a third copropagating laser beam at 0.526 µm. A novel use of Moire deflectrometry ensured the pump beams and probe beam were copropagating at focus. The inferred plasma wave density modulation δn/n is about 1–2% and corresponds to a longitudinal electric field of about 109 V m−1. The plasma wave was observed to exist for a time τp ≤ 50 ps. These results are not in agreement with theoretical predictions for relativistic detuning which give δn/n ≈ 8%, τp ≈ 200 ps and suggest that there is some mechanism limiting the growth of the plasma wave. Simulations indicate that this is due to either ponderomotive blowoff taking the plasma off resonant density or to the modulational instability.