As with most animals in films, several cats were used. Unfortunately, the cats used were not a good match for each other: while both are red tabby cats, one of those used is a "classic" tabby with markings that show a "bullseye" on its side, and another used is a "mackerel" tabby with stripes on the side.
Author of the book "Thomasina" Paul Gallico was present on the set during filming by special invitation. As Gallico quickly came to dislike "the great god Disney", he was vastly amused when one of the felines portraying Thomasina held up filming for two days when she flatly refused to perform a stunt for which she had been trained, in spite of Walt Disney's frustrated bellowing. Paul Gallico recalled in his memoirs: "I was proud of that cat!" As with many films with animals, more than one cat was used to portray Thomasina. Disney put a foreword on the film (DVD special features) and indicated 'Paul Gallico' was his friend.
During a scene in which Dr. McDhui is performing surgery, he asks his assistant for the "Spencer Wells". This is a type of forceps, named for Sir Thomas Spencer Wells, physician to Queen Victoria and a pioneer in abdominal surgery.
The film was first broadcast on television in three one-hour installments over three weeks on Walt Disney's Wonderful World of Color.
The song sung by the children - and, moments later, by Lori - at Thomasina's funeral is "The Bonny Banks o' Loch Lomond".
With the exception of Thomasina herself, all the cats in Cat Heaven appear to have been transformed into Siamese.