Level anticrossings and quantum beats have been observed for the fluorescence from single rotational levels of the vibrationless S-1 pyrimidine in a supersonic free jet. The density of triplet levels (rho) and the coupling matrix elements (upsilon) deduced from the data obtained for R(0) transition confirm that the singlet-triplet coupling belongs to the weak coupling limit (upsilon rho much less than 1) of interstate interaction, in which electronic relaxation from the prepared singlet state to the triplet manifold does not occur in the absence of collisions.