From a sample of 1240 ± 51 D+ <FONT FACE=Symbol>®</font> pi- pi+ pi+ we find gamma(D+ -> pi- pi+ pi+)/gamma(D+ -> K- pi+ pi+) = 0:0329 ± 0:0015 <img src="http:/img/fbpe/bjp/v30n2/10eq01.gif" border="0" align="absmiddle">. Using a coherent amplitude analysis to t the Dalitz plot of this decay, we find strong evidence that a scalar resonance of mass 483<img src="http:/img/fbpe/bjp/v30n2/10eq02.gif" border="0" align="absmiddle"> ± 17 MeV/c² and width 338<img src="http:/img/fbpe/bjp/v30n2/10eq03.gif" border="0" align="absmiddle"> ± 21 MeV/c² produces a decay fraction of approximately one half.