Fired this summer as the jockey for Chief's Crown, Don MacBeth got his job back today because bigger-name riders had other commitments in the Marlboro Cup. Then he proceeded to get the sweetest kind of vindication.

MacBeth rode a flawless race and drove the 3-year-old to an upset victory over the strongest field to contest a race in the United States this year. Chief's Crown scored by a neck over Gate Dancer, with Vanlandingham third. Greinton and Track Barron, considered the best in the West and East, respectively, finished out of the money.

If this was redemption for MacBeth, it was the same for Chief's Crown. The colt had started the year as a highly acclaimed champion and the favorite for the Triple Crown series. He suffered disappointing defeats in all of them, and MacBeth took the heat for an ill-judged ride in the Preakness. He was replaced first by Angel Cordero Jr., then by the equally illustrious Laffit Pincay Jr.

But because Cordero was committed to Track Barron and Pincay to Greinton, trainer Roger Laurin reinstituted MacBeth. It was a fortunate choice, because MacBeth had been riding the Belmont track sagaciously, understanding that the rail is a great disadvantage here. Abetted by his No. 8 post position, he was able to keep Chief's Crown in the best part of the track -- the far outside.

Vanlandingham and Track Barron went out to duel for the early lead, as expected, while Greinton stayed close on the rail after breaking from post position No. 1. Pincay found himself in an impossible position with the 9-to-5 favorite. "The trainer and a lot of other people throughout the day kept telling me that the rail was dead and to keep off it," he said. "We just had no place to go."

MacBeth had no such problems. He stayed wide in the middle of the pack and when Gate Dancer accelerated outside him on the turn, he said, "We let him go by and then eased outside him on the turn." MacBeth wanted to be the widest of all, and that's where he was as the field entered the stretch.

The two leaders were weakening and both Chief's Crown and Gate Dancer charged into contention. Meanwhile, Greinton was having more problems. Pincay tried to get him off the rail, but when he succeeded he found himself in heavy traffic. "When I left the rail," he said, "I got boxed in and couldn't move anywhere. I had a lot of horse under me, and I would have done a lot better if I'd had the room."

But by the time he got the room, it was too late. Chief's Crown had taken command of the race in midstretch, and he was holding Gate Dancer safe. When Greinton finally got back into gear, he could finish only fourth, a little more than a length behind the winner.

Chief's Crown covered the 1 1/4 miles in an unspectacular 2:01 1/5. He paid $23, $11.20 and $6. Gate Dancer returned $9.80 and $7, and Vanlandingham paid $8.60 to show.

The upset threw into turmoil the competition for various championships that the Marlboro Cup had been expected to clarify. Chief's Crown figures now to challenge Spend a Buck for the 3-year-old championship, but Laurin said he had loftier goals for his colt. "I'd rather have horse of the year," he said. "I think he is the greatest horse in America today." In fact, any definitive assessment of the best horse in America probably will have to wait until after the Jockey Club Gold Cup and Breeders' Cup.

Solo Native, touted as a "mortal lock" by Andrew Beyer in the ninth race at Belmont Park today, didn't live up to his billing. He finished fourth as the 8-to-5 favorite.

Pincay kept the California thoroughbred last in the 12-horse field during the early stages of the grass event, then tried to rally on the inside. But the colt accelerated ineffectually, and was soundly beaten by the 14-to-1 winner, Double Alias.