Veteran Paul Mosca scored a hole in one on the par-3 13th hole and shot 70 yesterday to tie first-round leader Mark Alwin with a 36-hole total of 139 heading into today's final round of the Maryland Open golf tournament at Bethesda Country Club. Alwin added 72 to an opening-round 67.

Mosca, head pro at Font Hill, a par-3 course in Ellicott City, hit a six-iron tee shot on the 156-yard 13th hole. The ball hit on the front of the green, took one hop and fell into the cup for his first competitive ace and his fourth overall.

"I hit a smooth six-iron," said Mosca, who played the two back-nine par-3 holes in a total of three strokes.

The ace allowed Mosca, who had struggled on his front-nine 37, to pull within two shots of Alwin.

On the next hole, the 388-yard par-4 14th, Alwin took double bogey when he pulled his approach shot under some pine trees to the left of the green and dumped his chip shot into a green-side bunker. It took Alwin three more shots to get into the hole from the sand. Mosca missed a six-foot birdie putt, but pulled even with Alwin with a par.

Alwin then regained a two-shot lead on the next hole when he sank a 35-foot birdie putt and Mosca took bogey. Mosca's tee shot on the 15th hole was not quite long enough to get around the dogleg and he tried to fade an iron shot to the green. However, the ball stayed left and stopped in high weeds. Mosca chipped his third shot past the pin and off the green, but chipped up and made a saving six-foot putt for bogey.

On the par-3 17th hole, Mosca used his cross-handed putting style and sank a 50-foot birdie putt to draw him within one shot of Alwin.

Alwin, who had been using his one-iron off most of the par-4 tees, used it again on the par-4 18th hole, but hit the ball into pine trees only 175 yards off the tee. Stymied by a thick tree only three feet in front of his ball, Alwin was forced to chip his second shot sideways to the fairway. He still was 190 yards short of the green, which he hit with his third shot and two-putted for bogey. Mosca, meanwhile, hit a good drive, a seven-iron 25 feet short of the hole, and two-putted for par and his share of the lead.

Bill Winters of Robin Dale, who started the day one shot behind Alwin, survived a front-nine 39 and birdied the 16th and 17th holes to be three shots back at 142. Two-time champion Larry Ringer shot 79--149. Amateur Gary Marlowe of Woodmont was one shot behind Winters at 73--143.