  We report the observation of commensurability oscillations in an AlAs
two-dimensional electron system where two conduction-band valleys with
elliptical in-plane Fermi contours are occupied. The Fourier power spectrum of
the oscillations shows two frequency components consistent with those expected
for the Fermi contours of the two valleys. From an analysis of the spectra we
deduce $m_l/m_t=5.2\pm0.5$ for the ratio of the longitudinal and transverse
electron effective masses.
