They're back in southeastern Alberta.
The City of Medicine Hat says that several rats have been spotted in its landfill.
The rats were spotted after a report came in about a single rat found in a farm yard south of Highway 41A.
The city, along with Cypress County and Alberta Agriculture are working together to investigate.
In August 2012, an 80-metre-long rats' nest was found in the landfill and residents called in sightings, resulting in at least 100 Norway rats being killed by city staff.
It took six hours for 21 workers and two excavators to dismantle the nest.
Since then, the landfill has been continuously monitored and the city credits that for discovering the new cases.
The city says additional bait stations have been put out to poison rats, and staff are checking sites daily.
Agricultural fieldmen known by Albertans as the "rat patrol" have worked for years targeting invading rats within a control zone along the province's eastern boundary.
Pet rats are also forbidden under provincial law.
(The Canadian Press)
Note to readers: This is a corrected story. A previous version said southwestern Alberta.
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