As is well known, the action of ultrashort resonant laser radiation pulses is able to induce coherent states of excitons / I / . The phenomena of self-induced transparency (SIT) can then be observed in the exciton part of the spectrum. The possibility of SIT due to excitons has been object of intensive study in the last years /1 to 4 / . In a very interesting paper Moskalenko et al. / 5 / studied the possibility of SIT due to excitons in semiconductors and showed that the existence of bleaching and darkening waves is determined by the sign of the exciton-exciton interaction constant g and concluded that g must be positive, i.e., a repulsive interaction constant. In solving the coupled equations that describe the excitonphoton system Moskalenko et al. 151 ignore the second derivatives with respect to x of the amplitude2 of the exciton and the photon fields. With this approximation they find erroneously that the exciton-exciton interaction constant g must be positive. In this note we show that solving the equations that describe the inhomogeneous states of excitons and photons in a rigorous way, the condition for SIT is g < 0 or the exciton-exciton interaction must be attractive. W e start from the coupled equations for excitons and photons and assume that the amplitudes of the excitons a and of the positive frequency part of a coherent electromagnetic field E+ depend only on the coordinate x and time t / 5 / , 2 2where Ql is the limiting frequency of transverse photons, m is the translational mass of the exciton, g is the exciton-exciton interaction constant, d is 'the dipole moment of the transition from the ground state to the exciton state of the crystal, c is the velocity of light in the crystal, and a = 4nd/voc2 with vo being the volume of the unit cell. Now to solve the system (l), ( 2 ) we take the solutions of such equations in the form a = $(x, t)exp(i(Kx -wt)) ,) 50739 Recife, Pernambuco, Brazil.