Team Ireland swimmer Shane Ryan is out of his best event, the 100m backstroke, due to injury.
Pennsylvania-born Ryan was due to start his Olympics in the heats on Sunday but has had to withdraw due to persistent shoulder pain, specific to the particular stroke.
He is a World and European Short Course medallist in the backstroke.
The decision was made by Ryan after consulting with the Team Ireland swimming and medical staff on the ground in Tokyo.
However it is hoped that the 27-year-old will be able to compete in the 100m butterfly and the 4x200m freestyle relay in the coming days.
Two Irish swimmers will be in the pool tomorrow, with Ellen Walshe competing in the women's 100m butterfly and Darragh Greene in the men's 100m breaststroke.
