Walsh world record, Ledecky shines at US Olympic trials

The US Olympic swimming trials have begun with a bang, with Gretchen Walsh setting a world record in the women's 100m butterfly a month before the Paris Games.

And minutes after Walsh's high-speed heroics, US great Katie Ledecky sent a message of her own to the swimming world - including Australia's Ariarne Titmus - by winning the 400m freestyle.

Walsh, who led from the outset, looked up in disbelief after she touched the wall in 55.18 seconds.

Her performance, in a semi-final heat, beat the previous record of 55.48 set by Sweden's Sarah Sjostrom in 2016.

"I definitely was feeling it," Walsh, 21, said.

"I thought I was dying. I didn't know I was going that fast - apparently, I took it out too fast.

"I just knew I wanted to go a fast time and now here I am - a world-record holder."

The trials will determine the US team for Paris and are being held over nine nights at the Lucas Oil Stadium, home of the NFL's Indianapolis Colts.

Seven-time Olympic gold medallist Ledecky qualified for her fourth Games with victory in the 400m freestyle.

Ledecky finished second over the same distance to Titmus at the Tokyo Olympics last time out.

The Australian showed her form with a world-record time of one minute 52.23 seconds in the 200m freestyle at Australia's Olympic trials last week, but Ledecky is confident she can move one step higher on the 400m podium in Paris.

"I never imagined I'd get to go to a fourth Olympic Games," the US swimmer said.

"It's going to be a great field.

"I've always done a pretty solid job in the period between the trials and the games at being faster at the games.

"The goal is no different this time around. Hopefully, that will put me in a good position in the 400."