Poison Gas Monster Appears (毒ガス怪獣出現 Doku Gasu Kaijū Shutsugen) is the eleventh episode of Return of Ultraman.
A filming team met an accident during filming on location. Kishida was horrified, after he heard the accident was caused by the Yellow Gas, a secret weapon Kishida's father once helped develop for the Armed forces of the empire of Japan. The capsule that contained Yellow Gas, which was supposed to be disposed, was eaten by monster Mognezun ate and it spitted out gas. Kishida prepares to fight Mognezun alone.
- Hideki Go (郷 秀樹 Gō Hideki):Jiro Dan (団 時朗 Dan Jirō)
- Takeshi Minami (南猛 Minami Takeshi):Shunsuke Ikeda (池田 駿介, Ikeda Shunsuke)
- Ippei Ueno (上野一平 Uneo Ippei):Mitsui Hatashi
- Yuriko Oka (丘ユリ子, Oka Yuriko):Mika Katsuragi (桂木美加,Katauragi Mika)
- Captain Katsushiro Kato (加藤勝一郎) Nobuo Tsukamoto (塚本信夫, Tsukamoto Nobuo)
- Fumio Kishida (岸田文夫 Kishida Fumo):Nishida Ken (西田健, Ken Nishida)
- Return of Ultraman Vol.3 Episodes 9-12