INDIANAPOLIS – The 2017 Phillips 66 National Championships are underway at the Indiana University Natatorium.

Thirty Olympic medalists from the 2016 Olympic Games in Rio will be competing this week at Nationals, vying for spots on the U.S. roster for the 2017 FINA World Championships, to be held July 23-30 in Budapest.

Also in the mix will be a group of hungry up-and-comers looking to establish themselves as swimmers to watch in the next three years heading into the 2020 Games in Tokyo.

Here’s a look at some of the highlights from this morning’s prelims, and some things to keep an eye on in tonight’s finals:

Women’s 200m Butterfly

• Olympian Hali Flickinger posted the fastest time during prelims with a 2:08.43. Flickinger will look to win her second national title in this event tonight. She finished seventh in the 200m butterfly at the 2016 Olympic Games.

Men’s 200m Butterfly

• Olympic medalist Chase Kalisz topped the prelims field with a personal record of 1:55.60. He will look to win his second national title, and first in this event. Kalisz was a silver medalist in the 400m IM at the 2016 Olympic Games.
• Fellow 2016 Olympian Gunnar Bentz followed with a 1:55.72, while Pace Clark rounded out the top three qualifiers. All are former University of Georgia swimmers.

Women’s 100m Freestyle

• Mallory Comerford broke the national championships meet record with a time of 53.26.
• Simone Manuel earned the second-fastest morning swim with a 53.50. Manuel won gold in this event at the 2016 Olympic Games.
• Katie Ledecky kicked off her double today with a sixth-place qualification in prelims.

Men’s 100m Freestyle

• Zachary Apple posted a 48.14 to claim the top seed heading into finals.
• 2016 Olympians Townley Haas and Caeleb Dressel claimed the second and third fastest preliminary times, respectively.
• Veteran Nathan Adrian picked up the fourth seed heading into finals. He will look to claim his eighth national title in the event.

Women’s 800m Free

• Katie Ledecky, the reigning Olympic champion and world record-holder in this event, will be the odds-on favorite in tonight’s timed finals. She owns 17 of the top 20 performances of all time in the 800m free. The second seed for tonight’s finals is Ledecky’s Olympic teammate, Leah Smith, who finished sixth in this event in Rio.

• Those up-and-comers we were talking about? They will be out in full force tonight in the men’s 1500m free. The top seed is Stanford’s True Sweetser, who finished second in this event at the 2015 Phillips 66 National Championships. Also in tonight’s finals are National Junior Teamers Robert Finke, Andrew Abruzzo, Michael Brinegar and Aaron Apel, along with National Teamer PJ Ransford.