Tunneling magnetoresistance (TMR) is one of the most important phenomena for future spin-electronics devices. Here, we present very large TMR (>70%) in all-semiconductor magnetic tunnel junctions (MTJs), having (GaMn)As ferromagnetic electrodes separated by an ultrathin AlAs tunnel barrier.@footnote 1@ Trilayer heterostructures, (Ga@sub 1-x@Mn@sub x@)As(x=0.04, 50nm)/AlAs(d nm)/(Ga@sub 1-x@Mn@sub x@)As (x=0.033, 50nm), were grown on p@super +@GaAs substrates by low-temperature MBE. Mesa etched MTJs with the barrier thickness d ranging from 1.3nm to 3.0nm were fabricated, and showed clear TMR due to the change from parallel to anti-parallel magnetization of the two ferromagnetic (GaMn)As layers. Very high TMR ratios up to 75 % were observed at 8K for the junction with d=1.5nm. For d@>=@1.6nm, the TMR ratio was found to decrease with the barrier thickness. This behavior can be explained by calculations assuming that the wavevector k// of carriers is conserved in tunneling. This means that conventional Julliere's model is not valid in such epitaxial MTJs. Also, we have found that the TMR behavior strongly depends on the applied magnetic field direction, which is well explained by the cubic magneto-crystalline anisotropy of GaMnAs.@footnote 2@ Unlike the conventional MTJs made of polycrystalline ferromagnetic metals and an amorphous tunnel barrier, the present MTJs are all-epitaxial monocrystalline semiconductor-based junctions, which have the following advantages: (1) MTJs made of all-semiconductor heterostructures can be integrated with semiconductor circuitry. (2) Many parameters, such as the barrier height, barrier thickness, and Fermi energy of the electrodes, are controllable. (3) Introduction of quantum heterostructures, such as resonant tunneling structures, will be easier than any other material system. @FootnoteText@ @footnote 1@ Y. Higo and M. Tanaka, Physica E (2001), in press. @footnote 2@ Y. Higo and M. Tanaka, J. Appl. Phys. (2001), in press.