The bilateral cavities of the piriform fossa are the side branches of the vocal tract and produce anti-resonance(s) in the transfer function. This effect has been known for male vocal tracts, but female data were few. This study investigates contributions of the piriform fossa to vowel spectra in female vocal tracts by means of MRI-based vocal-tract modeling and acoustic experiment with the water-filling technique. Results from three female subjects indicate that the piriform fossa generates one or two dips in the frequency region of 4-6 kHz. Experiment for filling each cavity separately demonstrated that the spectral patterns vary greatly across the conditions for filling both, left and right cavities. The result suggests that the cavities interact with each other, and the phenomena contribute to determining spectral envelopes in the higher frequencies.