Betsy Rapp of the Starlit Aquatic Club in Fairfax, Va., has waited a long time since that June afternoon in 1976 when she finished fourth at the U.S. Olympic Trials. She failed to make the U.S. team by three-tenths of a second.

But that long wait came to an end Thursday in this suburb of Houston. The 18-year-old finished second in the 100-meter butterfly in the AAU National. It qualified her for the World Championship Games Aug. 18-28 in West Berlin.

Rapp finished behind Nashville Aquatic Club's Joan Pennington, who lowered the American record to 1:00.58. She turned in a 1:01.55 and has her sights set on breaking a minute.

The world record 59.46 is held by East German Andrea Pollack.

Rapp said visions of just missing another national team flashed through her head as she charged down the last 20 meters of her race. "I have finished one place out of making a national team so many times. I just put my head won and hoped."

By keeping her head down at the end, Rapp edged out Diane Johanningman 1:01.98 to earn her second trip to Germany this year.

Rapp came to the Strlit Aquatic Club in the fall of 1973 on the urging of a friend and her sister, Jenny, who was already on the team.

Holger Dietz, who had just taken over as coach of Starlit, began her grind to the top.

"That first year was just awful. I hated it. I don't know why but it was just awful," she said Friday night while resting up for her leg of Starlit's 800-meter freestyle relay which finished fourth.

But after that first year swimming became much less distasteful. She qualified for her first national championships in Cincinnati in the spring of 1975.

Dietz, 27, said, "When Betsy first came to Starlit she was just another uncoordinated 13-year-old."

But years of double workouts to wear her body down to the point of exhaustions the rest of her Starlit teammates has paid off for the freshman to be at the University of Florida.

Her toughest competition in the Washington area is Jenny, her 15-year-old sister. She finished ninth here in the 100-meter butterfly recording a 1:02.6.

The girls don't carry their fierce competition from the pool to their relationship and actually reinforce each other 100 percent, Dietz said.

Betsy said of her relationship with Jenny: "When we're racing it dows not matter who she is. I'm out to beat her and she is out to beat me. When we're done, we're sisters agains."

Their father, Ed, is an Air Force officer. The Rapps moved to Alexandria from West Point, N.Y., in 1973.

Betsy has represented the United States in competitions in Argentina and in France prior to her trip earlier this year to East Berlin. But none offered the competition of a world championship.

The World Championship are expected to give some indication of the role the U.S. owmen will play in West Berlin and in Moscow in 1980.

And there is an excellent chance that two sisters from Alexandria, Va., will be on that 1980 Olympic team swimming the same event - and backing each other all the way.