2-(2-Oxo-1,2-dihydro-3H-indol-3-ylidene)acetic acids esters reacted with phenylhydrazine yielding products of the regioselective addition of the latter in the alpha-(C-2)-position of the exo ethylene bond, (2-oxo-2,3-dihydro-1H-indol-3-yl)(2-phenylhydrazino)acetic acids esters.