In an attempt to synthesize an indole derivative, methyl 5-nitro-lH-indole-2-carboxylate, an isomeric change of methyl 2-[2-(4-nitrophenyl) hydrazono] propanoate from E to Z geometry was observed. The two isomers were determined by single-crystal X-ray diffraction analysis. The Z isomer is stabilized in a six-membered ring conformation constructed by an intramolecular hydrogen bond. This isomeric change added a branched pathway in the mechanism of Fischer indole synthesis.