Phonon mediated electromagnetically induced transparency(EIT)and ultraslow light in strongly coupled exciton-phonon systems are investigated theoretically without making any approximation for excitons. It is shown that electromagnetically induced transparency mediated by phonons and the ultraslow light can occur in such systems as the signal field detuning from the exciton frequency is exactly equal to the phonon frequency. It is further shown that the phonon mediated EIT and ultraslow light calculated by using the exact exciton model is essentially the same as the numerical results obtained using the Bose approximation or the Dyson-Maleev approximation.