Michael Phelps took out the 100-metre butterfly at the United States team trials and will defend his Olympic crown in Rio de Janeiro.

Phelps, the three-time defending gold medalist in the event, swam a time of 51.00 seconds to win in Omaha on Saturday, qualifying for his third individual event at the Games.

Tom Shields finished second (51.20), just 0.04 better than top seed Seth Stubblefield, to clinch a spot beside Phelps in what will be his first Olympics.

Phelps, likely swimming on American soil for the final time in his storied 16-year career, will square off against international rivals Laszlo Cseh of Hungary and Chad Le Clos of South Africa in Rio.

Phelps edged Le Clos by 0.23 seconds for gold in the London Games in the 100 fly and the two have maintained a healthy competitive streak since.

Cseh is the owner of the world's fastest 100 fly time this year (50.86), but Phelps has the world record at 49.82.

Only four Olympians in history have won gold medals in the same event in four consecutive Games, and Phelps will have the opportunity to become the fifth.

He would also be the first swimmer to accomplish the feat.

There was some doubt though that Phelps would not qualify for the event after he swam a slow 51.83 in the semi-finals.

But that followed his showdown with Ryan Lochte in the 200-metre individual medley qualifier in which he defeated his countryman by 0.31 seconds.

Phelps also will compete in the 200 IM and 200-metre butterfly, his signature event, in what will surely be his fourth and final Olympic tour.

Shields will join Phelps in the 200 fly at Rio.