Kansas City, Mo ? Ameristar has surpassed Harrah’s as Kansas City’s casino market leader, taking its biggest bite ever in November from the local market-share pie.

Rival Harrah’s, meanwhile, posted one of its weakest months in history.

The two casinos routinely have competed for the biggest share of the market, and each has held the lead for about half of the past year. November was the second consecutive month that Ameristar had topped Harrah’s.

According to Missouri Gaming Commission reports, Ameristar grabbed a record 37.3 percent share to Harrah’s 32.7 percent. It was just the ninth time since 1997 that Harrah’s monthly share fell below the one-third mark — but the second time this year.

Now the playing field is changing, and such lofty market-share levels may not be seen again.

Thursday’s opening of a new and much larger Argosy Riverside Casino presents Harrah’s and Ameristar with a new rival, and they know it.

“When you compare it to what they had, it’s just a huge leap for Argosy,” Ameristar general manager Dave Albrecht said of Argosy’s $105 million, Mediterranean-village themed casino, which opened to rave reviews for its dramatic design and decor.

“I think it’s just awesome,” said player Trina Mock of Independence, Mo. “I love it. It’s different. It brings in the Las Vegas casino feeling.

“People will want to come back.”

And that is the main thing.

Argosy’s inventory of 1,750 slots and its proximity to Kansas markets poses a challenge to Harrah’s 2,012 machines. Meanwhile, Harrah’s appears to be trimming its slot count, which is down more than 30 since January.

Before the new Argosy opened, Ameristar and Harrah’s were near equals and natural rivals, as were Argosy and Isle of Capri.

Argosy’s new facility doubles the size of its now-closed riverboat. Along with a recent 400-slot expansion at Isle, market capacity has been bumped by nearly 15 percent, to 9,646 gambling “positions” for players.

In the new shakeout, Ameristar remains the big dog with 37.1 percent of total capacity, down from its previous 42.7 percent level.

Harrah’s falls from 27.9 percent to 24.3 percent, and Argosy jumps from 15.5 percent of total capacity to 20.1 percent.

Isle of Capri grows from 13.9 percent to 17.8 percent with its additional slots.

If each casino — and especially Argosy — can keep its positions filled with players, the overall market will grow.

If not, look for a marketing war next year that will offer giveaways to lure players.

“We’ll see if Argosy increases the market, or if one casino is just going to be stealing business from other properties,” Albrecht said.

“That’s the $64,000 question. It’s all a guessing game,” he said.