It says staff, along with police and fire crews, took 1,123 animals from the property in northeast Calgary on Tuesday.
Most of the animals were domesticated mice, along with three dogs, three cats and 80 fish.
Many of the mice were in poor health and 275 animals had to be euthanized.
A health inspector deemed the property uninhabitable due to unsanitary conditions.
Charges are pending.
“By volume, this is the largest animal seizure in Calgary history," said Brad Nichols, senior manager of cruelty investigations for the humane society.