The first thing you hear is the steady thumping of the tom-tom. Just ahead is a huge, grassy field, ringed by teepees. Indian men in feathered headdresses and women and children in fringed suede dress dance to the steady beat.

Amazingly, this is Cape Cod. And this July 6 and 7, the Wampanoag Indians of Mashpee, Mass., will hold their 50th annual powwow. Every year, the event draws several hundred Indians from as many as 15 different tribes across the country -- and hundreds of tourists as well.

Mashpee is just off Rte. 28 between Hyannis and Falmouth, along the southwest coast of Cape Cod. The Wampanoags, now 1,200 strong in Mashpee with perhaps a few thousand more scattered around the state, call the powwow more accurately a pauwau, meaning a gathering.

This old Massachusetts tribe consisted of hunters and fishermen when the Anglo Saxons arrived in the 1600s. The Indians didn't choose sides when the strangers got involved in a war among themselves. So, in gratitude, the winning colonists set aside a reservation in Mashpee in the mid-1600s for the Wampanoags -- reportedly the first Indian reservation in America.

Some Wampanoags have remained in Mashpee ever since, though it's no longer a reservation -- it's legally classified a town. In the late 1970s, the Mashpee Wampanoags went to court in Boston and lost a case that would have given them the town of Mashpee, a growing summer resort. But the legal defeat didn't affect the Wampanoag's traditions.

The Mashpee Wampanoags didn't beat tom-toms originally. Drumbeats were traditional for the warring Western tribes, but not for the hunters of the East Coast. But the Mashpee Wampanoags have gradually adapted themselves to the customs of the other North American tribes. Adaptation is good for the powwow business, for one thing. And the Wampanoags play host at the annual powwow to all types of tribes, some of them historically warring and Western.

"So many tribes are being invited this year that some of the names I haven't heard before," said Vernon Pocknett, a Mashpee Wampanoag Indian in charge of the powwow committee, recently.

They've been holding their powwow for more than 50 years, taking time out only during World War II. And this year they've sent fliers out to more tribes -- about 300 -- than ever before. In 1984, the Wampanoags played host to Narragansetts from Rhode Island, Iroquois from New York State and Blackfoot Indians from the Plains. In past years, Mohawks, Sioux and Cherokees have shown up, too.

Normally the powwow starts with chiefs of the various tribes smoking the peace pipe together. "Actually it's a calumet. Peace pipe is the white man's word," says Earl Mills Sr., Chief Flying Eagle of the Mashpee Wampanoags and owner of the Flume restaurant and fish market in Mashpee.

Last year, as usual at the powwow, the drummer beat steadily while members of all the tribes danced in the field. The drum was about four feet in circumference, and five young Indian men beat it with drumsticks that had fluffy felt tips representing several tribes. A sixth man held a microphone -- a distinctly modern touch -- over the drummers so that the entire field of people, both Indians and tourists, could hear and feel the full force of the tocsin.

Some men wore the long Indian headdresses of the Western tribes; some wore big raccoon hats designed for life in the Yukon; some wore loincloths as part of great feathered costumes. The women did simple folk dances, dressed in fringed suede dresses and moccasins. Indian children joined in, too, dressed to the nines.

That show should be repeated this summer. The teepees ringing the field will be authentically decorated with deer and reindeer paintings and vented at the top and the flaps. Scores of vendors' stalls will sell Indian crafts and jewelry, some of it at bargain prices. (In 1984, Zuni rings sold for $25; turquoise and silver rings started at $12 and went up to $35; beaded moccasins and belts were $12 with the name of the tribe, $10 without.)

The food booths sell stuffed quahogs (older clams too tough to eat by themselves but chopped and blended with other ingredients into a tasty me'lange); and codfish and potato dinners, served with fried pork strips and corn meal dumplings. There's usually a fried rabbit dinner, and bowls of jag -- rice and beans -- and clam chowder.

And it may look like soup, but it's dessert: plum porridge, a light, sweet Wampanoag pudding. You can find an elegant recipe that has evolved and made its way into a traditional Cape Cod recipe book sold in supermarkets. But the simple Wampanoag way is to blend cinnamon, sugar, raisins (they're the plums), canned milk, water and flour.

Rain can wreak a little havoc with the outdoor schedule. Last year's traditional Indian wedding festival scheduled for the powwow opening was rained out, as was the peace-pipe ceremony. Normally these types of celebrations start the day, followed by all kinds of dances: some in welcome, some meant to ward off evil spirits. Sometimes the Iroquois play soccer with a flaming ball. In 1984 they sent their Woodland Singers, a singing society formed in Niagara Falls, to help raise money for the Mashpee Wampanoags.

It's a commercial venture, yes, but with enough colorful and spiritual overtones to merit a special side trip from the better-known Cape Cod resorts.

On the second day of the powwow, always a Sunday, there's a noontime Christian ceremony (nothing in the Wampanoag language -- that's been lost for hundreds of years). The service is held in the Old Indian Meeting House fronted by a cemetery. The building dates back to 1774, replacing the older one that was built in 1684.

After the ceremony, everyone goes back to the ballfield for the clam bake -- lobsters, steamers, corn, potatoes, sausages cooked the traditional way, on heated rocks and seaweed. And the last dances are performed.

If you want to stay in Mashpee at powwow time, early reservations are advised. Try the 200-room New Seabury Resort (Box B, New Seabury, Mass. 02649, 617-477-9111); rates (two-night minimum) range from $90 a day double for a one-bedroom suite to about $160 double for a one-bedroom villa. Or try the 20-room Wigwam (Box 203, Mashpee, Mass. 02649, 617-477-1900) -- $35 a day double, also a two-night minimum. For powwow and other information about the Indians, write or call the Mashpee Wampanoag Indian Tribal Council, Mashpee, Mass. 02649, (617) 477-0208. Gate admission fee for the 1984 powwow was $3 for adults, $1.50 for children; this year's prices have not been set.