Middlesex County OPP say one person suffered life-threatening injuries in an ATV rollover in Thames Centre on Saturday afternoon.
Emergency services responded to the incident along Oliver Drive around 3 p.m.
The lone rider was transported to a local trauma centre.
OPP say the Traffic Incident Management Enforcement team is on scene to assist with the investigation.
Anyone with information is asked to contact the OPP at 1-888-310-1122 or report anonymously to Crime Stoppers at 1-800-222-84777 (TIPS).