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Energy transfer of the lowest triplet state of isotopic mixed naphthalene $h8$-naphthalene $d8$ single crystal was studied. To monitor the exciton flow, spectrally resolved phosphorescence time evolutions of guest naphthalene-$h8$ and trap beta-methyl-naphthalene, as well as other impurity traps, were measured at $4.2\circ K$, using a high pressure Xenon lamp as the excitation source. We find changes in the decay times around the triplet exciton critical concentration (about 9% naphthalene-$h8$ in -$d8$). This is consistent with the onset of multistep energy transfer in the guest (naphthalene-$h8$) component at this concentration. These results will be discussed in terms of the dynamic percolation and Anderson localization models.