More than 70 drain covers were stolen across Luton last weekend by thieves posing as highways maintenance engineers.
It was the third spate of thefts across the borough since January and the total number stolen stands at about 150.
Luton Borough Council said it believed the thieves could be posing as highways maintenance engineers.
The estimated cost of replacing the drain covers is about £37,500, a council spokesman said.
Bedfordshire Police are also investigating thefts in Crawley, Stopsley and Wigmore.
Graham Turner, highway services manager, said: "While the thefts seem to be confined to the Luton area, we know from previous experience that the criminals responsible can quickly move on to other parts of the county. So we're asking all of Bedfordshire's residents to be vigilant."
Dave Taylor, executive member for highways, said: "They are probably being stolen for their scrap value and these thefts mean we are having to use valuable resources to replace them."