In his skivvies, Morbulus climbed into a drain pipe where he encountered Dr. Viper, who took the defeated villain back to his lab in Megakat Swamp. There, Morbulus offered to help the doctor steal Katalyst 99. In order for him to do so, Dr. Viper made him a "living test tube" for his new bacteria strain, mutating him into a hulking purple bacteria blob which divides in two, like an amoeba, when hit, and again so each monster had one eye. The Swat Kats eventually destroyed all the bacteria monsters with electricity, but not before the monsters had eaten a farmer, a cow, Dr. N. Zyme, and a subway car full of passengers.
Morbulus was voiced by Jim Cummings in his only appearance. According to Mark Lungo’s article on the series, Morbulus was originally going to be called “Occulus,” until it was discovered that there was already a Marvel villain with that name. As a result, his name was changed to Morbulus, although he is still identified as Occulus in the model sheets and storyboards for "The Giant Bacteria."