Mrs. Frisby is the protagonist of the book Mrs. Frisby and the Rats of NIMH. She is a widowed field mouse who lives with her 4 children, Martin, Teresa, Cynthia and Timothy in a cinderblock in the fields of the Fitzgibbon farm. Her deceased husband is Jonathan Frisby, who was killed by drugging the farmer's cat, Dragon. Ultimately, she assists the mysterious rats of NIMH in their plans to move out of the farm and stop stealing its resources in return for helping her to move her sick son Timothy.
- In the film, The Secret of NIMH, Mrs. Frisby was renamed Mrs. Brisby for fear of copyright issues with the Frisbee toy.