The Effect of Gamma Rays on Man-in-the-Moon Marigolds is a 1972 American drama film produced and directed by Paul Newman.

Ruth is a rebellious adolescent who has epilepsy, while shy Matilda, highly intelligent and idealistic, seeks solace in her pets and school projects, including the one that provides the film's title.

Matilda's science experiment is designed to show how small amounts of gamma radiation from cobalt-60 affect marigolds; some die, but others transform into strange but beautiful mutations completely unlike the original plants.

Although the story was set in Staten Island, New York, director Newman said that he chose to shoot the film in Bridgeport, Connecticut because it was only 17 minutes from his home in Westport.

"[4] Time Out London called the film "an engaging adaptation" of the play "which sees Paul Newman's cool, lucid direction transforming what could have been a pretentious domestic drama into a touching account of small joys in sad and stunted lives ... Potts steals the movie, but what makes it so watchable is Newman's reluctance to sentimentalise.

[6] Joanne Woodward was nominated for the Golden Globe Award for Best Actress – Motion Picture Drama but lost to Liv Ullmann in The Emigrants.