Multiphoton up-down-up-...(UDU) excitation in a two-level system was realized under adiabatic conditions. Spectra of C2H4 excitation were investigated and UDU peaks were observed. The dependence of peak amplitudes on the powers and the frequencies of exciting fields were investigated experimentally as well as by computer simulation based on the rotating wave approximation (RWA). Agreement between experimental and theoretical results was observed.