Previously undetected autoionizing Rydberg levels of ammonia which exist in the energy region between the first ionization continuum and the first excited vibrational level of the NH3+ ion have been measured. Jet cooled ammonia molecules in high lying Rydberg levels were detected by a two-color, three-photon excitation resonantly enhanced multiphoton ionization process, 2+1’. The Rydberg series can be assigned to a continuation of the (a"(2))(nsa’(1)) series to which the (A) over tilde, (D) over tilde’, and (E) over tilde" states belong. A vibronic autoionization mechanism based on the electrostatic and linear terms [V-0, and (partial derivative V/partial derivative Q)Q, respectively] in the potential energy expansion has been suggested.