A time-resolved ENDOR spectrum of the triplet state of the primary donor of photosystem II in isolated reaction centers has been obtained at cryogenic temperatures. Selective excitation of donor molecules, which have their z triplet axes parallel to the magnetic field, has permitted the measurement of the Azz component of the hyperfine coupling tensor of protons, in the reference frame of the zero field splitting tensor. For comparison, measurements have also been made on the photoexcited triplet state of monomeric chlorophyll a in vitro. The results indicate that in the temperature range 10-20 K the triplet state of the primary donor of photosystem II is largely located on a monomeric chlorophyll a molecule.