All-optical phase modulation in single crystal waveguides of a polydiacetylene has been investigated by two different interferometric methods: Mach-Zehnder and Sagnac. Large phase changes (> (pi) ) have been observed in a short waveguide (2 mm) for a relatively low optical intensity (-10 MW/cm2), at the wavelength of 1.06 micrometers . These phase changes are due to the off-resonant nonlinearity having an ultrafast (< ps) response time. The results are important in terms of all-optical device applications of these materials. The resonant nonlinearities of these materials were previously studied in detail. The present research is focussed on the off-resonant nonlinearity the origin of which has not been understood as yet.