  Tokunaga et al. [Phys. Rev. B Vol. 59, R7837 (1999)] claim the first
successful observation of the inverse exciton M series in the emission spectrum
of excitonic molecules. We assert that actually such a series was observed as
early as 1989 in the beta-ZnP_2 crystal. We show that the objections of
Tokunaga et al. against the biexciton nature of the inverse exciton series in
beta-ZnP_2 are ungrounded. In particular, their estimations for the ratio of
the intensities of M2 and M1 emission lines in this crystal give two order
smaller value because they do not take into account the reabsorption of the M1
line photons. We report the observation of additional inverse exciton M' series
in the emission spectrum of excitonic molecules in beta-ZnP_2.
