STATE COLLEGE, Pa. — Iowa slugged out an ugly win in soggy Happy Valley to upset Penn State for the second straight year.

Adrian Clayborn returned a block punt for a touchdown and the Hawkeyes beat mistake-prone No. 5 Penn State 21-10 on Saturday night.

A year after a stunning 24-23 win knocked the Nittany Lions out of the national title race, the Hawkeyes (4-0, 1-0 Big Ten) took down Penn State (3-1, 0-1) again, this time in a drenching rainstorm.

"It was a team loss. No one person, or one play lost it," Penn State coach Joe Paterno said. "We got licked."

After falling behind 10-0 in the first quarter, Iowa's defense shut down quarterback Daryll Clark and the Penn State spread HD offense with withering pressure. Broderick Binns' sack led to a safety in the second quarter.

A defensive end, Clayborn rumbled 53 yards after blocking the punt to give Iowa an 11-10 lead with 12:21 left.

Clark was intercepted on the next drive to set up Adam Robinson's 13-yard touchdown run to put the Hawkeyes up 18-10 with 8:32 left.

It didn't get any better for Penn State.

A 36-yard punt return by Chaz Powell, plus a 15-yard personal foul penalty gave the Nittany Lions the ball at the Iowa 36 after Robinson's touchdown.

But tailback Evan Royster fumbled the ball away on the next play, appearing to lose control after trying to lunge for extra yardage.

"I thought the kids hung in there, tried to come back, and they kept making mistakes," Paterno said.

Clark was intercepted again with four-plus minutes left.

It was a disappointing end to a strong start for Clark, who connected with Powell for a 79-yard touchdown pass on Penn State's first offensive play of the game.

Later in the first, Penn State opened up the offense by running Clark on keepers and methodically marching down the field on a 20-play, 10-plus minute drive that ended with Collin Wagner's 27 yard-field goal for a 10-0 lead with 52 seconds in the first quarter.

After hitting his first five passes, Clark finished 12 of 32 for 198 yards with one TD and three interceptions.

Ricky Stanzi finished 11 of 26 for 135 yards and two interceptions for Iowa, and Robinson had 88 yards on 19 carries.

After opening the season with victories over three weak nonconference opponents, the Nittany Lions' offense failed its toughest test yet in the Big Ten opener.

On defense, Penn State star linebacker Sean Lee (left knee) didn't play, but the Lions got a boost from the return of fellow standout Navorro Bowman from a right groin injury. They held an opponent without a touchdown in the first half for the fourth straight game.

Iowa did manage 163 rushing yards on the Nittany Lions' D, yet the Hawkeyes couldn't convert into scores until late.

Their defense was just as stout.

Penn State followed up their nice first quarter with three straight three-and-outs, including Binns' sack of Clark in the end zone that led to the safety. Iowa outgained Penn State 124 to minus-6 in the second quarter.

Pat Angerer led the active Iowa defense with 14 tackles, one forced fumble and the 38-yard interception return that set up Robinson's TD.

The Hawkeyes ruined Penn State's "Whitehouse," in which the school asked its fans to wear white team gear. Over 109,000 people packed Beaver Stadium.

In the end, it was the loud Iowa fans in bright yellow clothing that stood out in sad Happy Valley.