Resonant tunneling and accompanying negative differential resistance are observed at 85 K in Al0.4Ga0.6As/GaAs triple barrier diodes, where two GaAs wells are separated by three AlGaAs barriers. Five resonance peaks, one small peak for one bias polarity, one medium and one large peak for each bias polarity, are observed. These are the resonance current peaks from the ground level of a populated well to the ground, the first excited or the second excited level of an adjoining unpopulated well. These are the direct electrical observations of the resonant tunneling between confined electron states in two potential wells of a semiconductor multiheterostructure.