Lincolnshire Police are warning cyclists in Lincoln after 69 bicycles were stolen in the city since the beginning of August.
Despite three arrests and increased police activity, including extra patrols, the trend is continuing, officers say.
Police believe both men and women are stealing the bikes and it is taking place at all times of the day and night.
Hotspots in the city seem to be Upper High Street, The Maze and St Catherine’s.
Sergeant Paula Gowler said: “We have made arrests and a number of bikes are recovered on a regular basis, but we often find we cannot return them to their owners due to poor descriptions of them when they are reported stolen.
“Victims very often haven’t noted down the serial number and very often they haven’t been security marked.”
Cyclists are being urged to makes sure their bikes are as secure as possible, to make sure they have a record of their serial number and to get their bike properly marked.
Shops are also being urged to be vigilant in terms of the bikes they buy for resale. Checks will be done and anyone found handling stolen goods will be prosecuted.
“If you own a bike, especially if it is a valuable one, then please do this,” added Gowler.
“If you want us to stamp or UV mark it then get in touch with your nearest beat team via the non-emergency number and we’ll do it for you.
“Apart from that, please ensure you have the best lock you can afford.
“If you run a business and your staff ride to work – please try to provide and storage area that is as secure as possible,” Gowler advised.