NEW YORK (AP) _ The two Japanese strolling through Manhattan's trendy SoHo section seem like ordinary businessmen until a young man in a sleeveless jersey strides passes them carrying a booming radio the size of a suitcase.

Suddenly, Katsuhiko Yamamoto whips around, reaches for his camera and snaps off a quick series of photos.

Yamamoto and his colleague, Osamu Akiyama, are on a mission. Their job: to track down American yuppies and report back to the home office of Sharp Electronics Corp. in words and pictures.

Sharp's ''town watcher'' project is believed to be the first major effort by a Japanese company to study American consumers in depth in their natural habitat.

Sharp hopes to customize its products to suit the lucrative but idiosyncratic U.S. market. And their main interest is in yuppies, who have the biggest appetite for the kind of upscale consumer electronics for which Sharp wants to be known.

''To make a target of yuppies is very popular in Japan now,'' Yamamoto said.

The Japanese, Yamamoto said, can no longer compete solely on the basis of price because of the yen's sharp rise against the currencies of the United States, Taiwan and South Korea since early 1985.

''We must find an advantage'' to survive the challenges of Taiwan and Korea, Akiyama said. Tailoring products to appeal to affluent American consumers is the answer, they believe.

The man with the suitcase-size radio this day was not, alas, a yuppie, but as the afternoon wore on, Yamamoto and Akiyama saw hundreds of young urban professionals.

With clipboard and camera, they browsed through open-air stalls of imported smocks, examined kelly-green knapsacks in the window of a sporting goods shop and checked out the display of the Rolling Stones' latest single in a record store.

''I'm an antenna for the company,'' Akiyama said between stops, speaking in halting English. ''What's new now is very important.''

At Shea Stadium, where Yamamoto and Akiyama caught a New York Mets baseball game, the men were intrigued to see some of the fans watching the action on tiny televisions instead of looking down at the diamond.

In California, Sharp's town watchers were invited into five homes in the Los Angeles area, where they snapped pictures of bathtubs, kitchen counters, beds and dressers.

A child in one home told them he had a ''small'' television, which turned out to have a 19-inch-diagonal screen. ''In Japan, 19 inches is a big television,'' Yamamoto said.

The town watchers also report about things that have no obvious connection with consumer electronics, such as the 13-year-old girl who told them she spent 13 minutes a day putting on makeup.

''We want to introducethe way of life in the United States to Japan, even such small things, and they can feel the atmosphere,'' Yamamoto said.

Their bosses back in Japan are especially keen on finding out more about yuppies, but Yamamoto and Akiyama are hard-pressed to come up with a neat profile. Everyone seems to want something a little different.

''There are many different people in the yuppie group,'' Akiyama said. In contrast, he said, Japanese preferences are rewardingly predictable.

''There is very nice quality control,'' he quipped. ''Like a clone.''

Although Akiyama is reluctant to take credit, he is said to be the man who thought of producing Sharp radios, televisions and other products in pastel shades instead of just the wood-grain or metallic look. Although not to everyone's taste, including his own, the colorful electronics have been big sellers.

Akiyama is manager of the design center at Sharp's U.S. headquarters in Mahwah, N.J., while Yamamoto is assistant manager for product planning in what Sharp calls its ''creative lifestyle focus center'' - the home of town- watching.

Sharp began town-watching in Japan in April 1985, about a year before the U.S. project, and the effort there already has produced one product, a microwave oven with a bar-code scanner.

Town watchers noticed that some Japanese, especially older women, were ''a little intimidated by the control panel'' on microwave ovens, said Daniel Infanti, Sharp's U.S. manager of corporate marketing and communications.

Sharp published a cookbook with bar codes at the bottom of each recipe containing the cooking instructions. The user runs a scanner over the bar code and plugs the scanner into a slot in the oven. Automatically the oven carries out the instructions: say, 3 minutes on high power, 2 minutes on simmer and 1 minute on medium.

Sharp still does all the things other consumer-products companies do - subscribing to market-research reports and holding in-depth interviews with consumers in focus groups - but town-watching gives a special dimension, Infante said.

''People are now so much more personally involved in their electronics. They want personal stereo headphones that are in tune with their image. I think that's all driven by the baby boomers,'' Infante said.

''For a consumer-electronics product to survive down the road, you have to do something a little different,'' he said. ''We feel this is a way for us to come up with something a little different.''

End Adv Weekend Editions Sept. 27-28