Younger brother Brett, 22, went first in the heats at the Aquatic Centre, finishing fifth in 1 minute 47.74 seconds. Shaune, 24, was eighth in his heat in 1:48:53.
New pool king Ryan Lochte, who trains with the Frasers in Florida, won Brett’s heat in 1:46:45. Lochte beat the great Michael Phelps in the 400m individual medley to win gold on Saturday.
Brett’s time was fast enough to reach the semis when he swam 1:47:01 to finish sixth. It was not fast enough to reach Monday’s finals but was his personal best and a new national record and was 12th fastest at the Games. Shaune’s time was 20th best.
The Frasers now swim in the 100m heats on Tuesday. Then Brett goes in the 50m freestyle and Shaune races in the 100m butterfly.
Donald McLean, president of the Cayman Islands Olympic Committee, said: “I’m very pleased. They both swam well. Brett did a little better than Shaune but more importantly, he set a new national record and a personal best.
“Although Brett missed getting to the final he still did well. This was the first time a Caymanian has reached an Olympic semi-final. And it’s not finished yet. There’s plenty more swimming to come.”
Doctor Sook Yin is the swimming manager. She said: “Having spent five days in the Olympic village moving amongst all these great Olympians, watching them train, eat and conduct themselves, made me realize the sacrifices that they have to make to get to where they are.
“Qualifying and winning really boils down to a hundredth of a second, a point or a goal. These athletes work hard to get to where they are and they are there to win and our two swimmers are no different. Both the Fraser brothers are focused, disciplined, highly competitive and never settling for mediocrity.
“In the 200m free on Sunday, Shaune swam hard but didn’t make the semis but he is still in the top 20 in a field of 48 of the best in the world in this event.
“Brett’s swim was strong in both his prelim and semis. He was ranked 14th after the heats and swam his personal best but he improved his ranking to become 12th in the London Olympics. No mean feat when they both come from a population like ours.
“I am so proud of what they have achieved and yes they are disappointed they didn’t make the finals as they really wanted to make Cayman proud. Judging by the number of phone calls and emails from well-wishers from Cayman, I think they can be assured that our nation is already very proud of them.
“Both of them have refocused and started preparations for their next race on Tuesday. No doubt our whole nation will be rooting for them.”