The first photon echo experiments on polymethylmethacrylate (PMMA) down to temperatures below 1 K are reported. A linear temperature dependence of the linewidth pf the chromophore zinc-meso-tetraphenylporphine (ZnTPP) is obtained below 1 K. Based on this result a flat distribution of two-level system energies E on the time scale of our experiment is deduced. Above I K an activated process with an energy of 5 cm(-1) is found, which explains the power law dependence usually measured by photon echo experiments in the temperature region 1.2-2.1 EC. The fit parameters of the proposed process are shown to be in agreement with hole burning measurements and with specific heat data on PMMA.