But here he was last month, in the birthplace of the atomic era, to deliver a lecture at the monthly Director’s Symposium. Nearby was a museum with Manhattan Project artifacts, and surrounding him were Los Alamos scientists who were curious about how this man, now 84 and a professor emeritus at the Marine Biological Laboratory in Woods Hole, Mass., felt about the bombings in 1945.

That is not, though, what he chose to address in his talk to about 100 Los Alamos scientists and lab workers.

Instead, he recounted the discovery and development of one of the most significant tools for modern biotechnology: the green fluorescent protein, or G.F.P., used widely in cell and molecular biology as a visual tracer. The discovery, which has deepened the understanding of a wide range of fundamental biological processes, brought him the Nobel Prize in Chemistry in 2008, along with Martin Chalfie and Roger Y. Tsien.

“I decided to focus on the science,” he said.

So it has been for Dr. Shimomura since shortly after Japan’s surrender. In the chaos after the war, he spent two years “idling,” he said, before enrolling in pharmacy school, a path that was not his first choice. But not long after, during a sabbatical in the 1950s, he began his life’s work, the study of bioluminescence.

He chose as his subject a crustacean sometimes known as seed shrimp that emits a striking blue light and is plentiful in the waters around Japan. His focus was on a class of compounds, luciferin, that are bioluminescent. After years of research at Princeton, they had still never been purified.