English rider Jamie Hodson died in a crash at the Dundrod 150 on Thursday while Northern Irishman Stephen Lynd is critically ill after another incident.
Hodson, 35, from Wigan died after the incident at the Joey's Windmill section of the Northern Ireland circuit.
His brother Rob Hodson was involved in the crash but was not seriously hurt.
Templepatrick man Lynd, 36, suffered his injuries in a crash on Wednesday while a third serious accident led to Thursday's Dundrod 150 being abandoned.
The condition of the rider involved in Thursday's second crash is unknown.
That incident happened in the National Challenge event and led to roads being closed for two hours as police investigated the incident.
The earlier National race was immediately red-flagged and the Hodson brothers were taken to hospital after being treated at the scene.
Jamie Hodson's death was later announced shortly after 22:00 BST.
An experienced rider, Jamie Hodson was the reigning Manx GP Supertwins champion, and this year achieved a top-10 finish in the Isle of Man TT Lightweight race.
The organisers of the event, the Dundrod and District Motorcycle Club, offered their sincere condolences to the Hodson family and friends and asked for their privacy to be respected.
Thursday's Dundrod 150 meeting was part of the Ulster Grand Prix meeting which concludes with Saturday's main day of racing at the circuit.