The theft of roadside grates in Nottinghamshire has cost £275,000 since the start of 2011, the county council said.
The authority said 693 cast iron grates had been stolen in 2011, which is 114 more than the same period last year.
Fifty-nine were taken in the Mansfield area alone during a two-week period in June.
It is thought that the offenders use pick-up trucks with flashing lights, so people believe they are carrying out highway repairs.
Councillor Richard Jackson, the county council’s cabinet member for transport and highways, said: “We want people to be vigilant.
“These gully grates are expensive to replace because, although the thieves mostly just take the grate and leave the frame behind, we have to buy the replacement as a complete unit.
“This is money which could have been spent on other highways works.”