The Dundrod 150 Superbike race was called off just a single lap in after poor weather conditions forced organisers into deploying an early red flag.
Rain, which has blighted the event so far, and low visibility had prompted many riders to raise concerns about the conditions, with Keith Amor choosing not to start at all.
Even so, the race got underway, with Guy Martin forging ahead to lead by nine seconds at the end of lap one from Ryan Farquhar. However, the worsening conditions would see several riders pull into the pit lane, prompting the red flag to be deployed, before organisers then took the decision to curtail the day's action altogether.
The abandonment came after Derek Costello crashed heavily during the National Race, the Skerries rider taken to Royal Victoria Hospital in Belfast, where he is described to be 'critically ill and under observation'.
The Ulster Grand Prix road races will be held on Saturday.