West Yorkshire Police are appealing for any information after a man exposed himself to a runner on the A65 between Ilkley and Burley-in-Wharfedale on Wednesday evening.
The incident happened around 20:45 close to the nature reserve on Coutances Way.
He is described as being of slim build, wearing dark clothing on top half and possibly a cap or hat.
Officers say neighbourhood teams have been conducting reassurance patrols in the area.
Police say: "Units attended the area within 15 minutes of the call and made an extensive area search. Unfortunately no one was located, but we will continue to monitor the area. If anyone has information, please get in touch via 101 quoting the log number 1645 from 16.03.22.”
Anyone witnesses any suspicious behaviour is asked to report it to police via 101 or use the live chat facility at westyorkshire.police.uk/