Boutiques have a funny way of letting you know they’re not just Any Old Store. Some make you ring a bell and wait forever before buzzing you in. Others choose to occupy lofts the size of eight Gaps and use them to carry, at most, three items per season. But perhaps the ultimate signifier that you have happened upon very hallowed shopping ground is when a clerk greets you with the question: “Have you shopped with us before?”

Make no mistake. Salespeople don’t say this because they think they recognize you. Even if they did, they’d be unlikely to admit it. Rather, they are trying to convey the message that the store employs its own elaborate customs, and if you don’t know what you’re getting into, you probably shouldn’t be getting into their fall collection.

Shopping for a swimsuit at the Mulberry Street flagship store of Malia Mills is quite the production. Grabbing something off the rack and running off to lunch simply isn’t done; most people clear an entire afternoon for the occasion. Customers are parked in little changing rooms, engaged in lite-interrogation (“Where are you going?” “Who are you going with?”) and encouraged to try on 20 versions of a bikini before making up their minds. Once a customer arrives at a decision, purchases are mummy wrapped in crepe paper and receipts are sealed off in big, creamy envelopes, which can only be an attempt to put off the buyer’s post-purchase guilt. Open the envelope and you’ll find a receipt for one of the no-frills swimsuits. They cost $200, more than eight times the price of the average bathing suit.

So the power suit is back; it just happens to be a bikini. In a city where the Look-at-Me set spends $1,300 for a handbag and half as much for a pair of sunglasses, there is great enough need for a top-shelf bikini to justify three Malia Mills boutiques in the city (the other two are on the Upper East Side and the Upper West Side). New York women pass on the designer’s name with the kind of secrecy usually reserved for admissions test tutors or miracle doctors. Fans say the suits are perfect – elegantly cut, well-fitting, durable. They say it’s worth every penny, even if the little wisp of clothing weighs less than a cashew and is bound to spend most of its public life soaking wet. “Nobody understands,” said Beth Blake, 34, a designer of bridesmaid’s dresses who has the triangle top and boy shorts in eight colors. “It’s like lingerie that really supports you. They’re classic, they fit, and they last in the water. Bathing suits are what Malia does. It’s not a little side thing.”

Still not sold? Madonna has one.

Malia Mills, 39, grew up in Honolulu, and studied fashion at Cornell. After apprenticing at Jessica McClintock, she launched her own swimwear collection out of her New York apartment in 1991. Three years later, Kathy Ireland wore one of Ms. Mills’s bathing suits in Sports Illustrated, allowing Ms. Mills to quit her job as a waitress at the Odeon. She opened her first store in 1997 in NoLita and she still co-owns her company with her sister, Carol Mills. The two spend a good deal of their bathing-suit time at Carol’s pool in Weston, Conn.

The company motto is “love thy differences,” and the philosophy is to give women swimwear that fits perfectly so they can stop tugging and readjusting. The cuts are straightforward, and don’t concern themselves with creating any optical illusions. They come in plain colors and a couple of paisley patterns, and not a single cut relies on pads or trick seams. The suits tell it like it is. The WonderBra of the sea they are not.

“If you’re in the right suit and the right size, you look your best anyway,” the company’s production manager, Meg Woodhouse, said. “It’s not about holding you in or being super-tight. It’s not smoke and mirrors.”

The company does brisk business even in the usually slow months between summer and the winter holiday season. “It’s always summer somewhere,” a saleswoman said the other day as she passed an armload of separates through a changing room curtain.

The store’s main bragging right is that it offers bikinis as separates, allowing customers to mix and match cuts and patterns, as well as buy ensembles that fit their bodies, even if their northern and southern hemispheres happen to come in different sizes. Specializing in separates is a brilliant move; according to the NPD Group, a market research firm, growth in swim separates was 10.6% last year, while regular bikini sales were down 4.7% for the same time period. The trend of buying separates is a natural extension of the popularity of customized fashion, like embroidered jeans or create-your-own sneakers. The buyer feels looked after, unique, even.

But it’s also testament to the power of persuasion. Jennifer, a film producer who lives in Brooklyn and did not want her last name used because she was worried that work associates might read this article and tease her for spending her time thinking about bathing suits, recalls shopping for a Malia Mills suit last spring. Everyone in the store was in an effusive mood, plying one another with compliments. She tried on every size and color and ended up being talked into buying a sea green two-piece by her fellow shoppers. “I felt like a million bucks,” she said.

It wasn’t until later in the summer when she put on her new suit for a day at the beach. Suddenly the bikini seemed too small, the fabric too thick, and she spent the day tugging at the edges and feeling self-conscious. “I wasn’t in a dressing room full of naked women encouraging each other to buy fancy bathing suits anymore,” she said. “I never wore it again. You can have it.”