We have made off-resonant nonlinear optical measurements of PTS polydiacetylene in a Fabry-Perot cavity. The measurement was made by a pump-probe technique using 90 ps pulses at 1.06 mu m wavelength. The change in transmission at specific bias points on the F-P fringe, was measured as a function of time delay between the pump and probe. Large (similar to 50%) changes in transmission with a pulse-with-limited response time, were observed at low optical intensities. The magnitude of n(2) as measured (similar to 10(-5) cm(2)/MW) and the negative sign are consistent with our Z-scan measurement of PTS at this wavelength. The results of the measurement on the F-P cavity clearly demonstrate possibilities of applications of PTS in ultrafast (sub-picosecond) all-optical switching and logic operations using this device geometry. This is the first time that all-optical switching with time resolution has been demonstrated for an organic material.