FM 97-8, chao-dyn@xyz.lanl.gov \# 9709011 mp_arc@math.utexas.edu \# 97-479 Author: G. Gallavotti Title: Fast Arnold's diffusion in isochronous systems Abstract: an illustration of a mechanism for Arnold's diffusion following a nonvariational approach and yielding polynomial estimates for the diffusion time. Keywords: Arnold diffusion, homoclinic splitting, KAM Fisica, Universita' di Roma La Sapienza, P.le Moro 2, 00185, Roma, Italia e-mail giovanni@ipparco.roma1.infn.it tel. 6-49914370, fax 6-4957697