The number of complaints in Sheffield about rats has doubled in the past seven years, with milder winters one of the main reasons.
It's a national problem, say city environmental protection officers.
"We used to have a high mortality rate for rats in cold weather but milder winters mean more are breeding," said Neil Wilkinson, of the council's pest control department.
"There has been an increase in fly-tipping and the dumping of food waste and there is also an issue with drains. Rats are found in sewers but they can break out through a defective drain or neglected public toilet."
The council received more than 5,000 complaints about rats last year. It also dealt with 2,144 calls about mice and 2,440 about insects.
The public is being urged to adopt "good housekeeping", particularly when it comes to disposing of waste food.
Mr Wilkinson added: "Prevention is better than cure but people should always ring us as we can carry out treatments in their back garden free of charge. The first thing to do if you see a rat is to report it.
"Tidy away garden rubbish and make sure your compost bin is on a hard surface so rats can't burrow under it. Don't compost fresh food, stick to green waste.
"Use a proper bird feeder and don't throw chunks of bread out. If you have an old outside toilet which is not used then seal it and close any gaps in pipes or holes."