Normally: No. But there are exceptions:
A4.14 ENEMY UNITS: Infantry may not move into the same Location containing an unconcealed enemy unit during the MPh [EXC: Berserk (15.43), Human Wave (25.23), Disrupted (19.12), Unarmed (20.54), and Infantry OVR (4.15)], but may do so during the APh. However, PRC (7.211, D6.5) and charging Cavalry (13.61) can dismount in such a hex during the MPh.
You should look up what happens if you try to enter the location of a concealed unit...