The man who walked into the Deak & Co. office at 18th and K streets recently to exchange $900 for Swiss francs had no intention of taking a trip to Europe.

"He didn't even know how many francs he'd get," recalled Deak Washington manager Charles Provine, repeating the customer's observation that "1,900 francs for $900 seems like a pretty good deal."

It was a pretty good deal. That $900 worth of Swiss francs purchased the first of February was worth$992 this week.

The man who bought them was one of the new breed of international currency speculators, little guys who have found a way to get into the world of international finance.

For investors who've been washed out of Wall Street, aren't rich enough for real estate, or are scared of pork bellies, currency speculation is the newest game in town.

There's no minimum investment required. Commisions are 1 percent or less on most transactions. And cash currency dealings of less than $10,000 don't have to be reported to the Internal Revenue Service: the profits aren't tax free, but they're tough to trace.

Although purchases of foreign cash are the simplest kinds of small-time currency speculation, buying travellers' checks in foreign denominations is more common, says Deak's Provine. Foreign currency travelers' checks are negotiable, replaceable if lost or stolen, and just as easy as cash to hide from the tax man.

Besides cash and travelers' checks, there are certificates of deposit in Australia banks, annuities payable in Swiss francs, and a handful of other weird ways to make money speculation in foreign exchange.

The Swiss franc is the speculator's favorite in Washington, with lesser play on the German mark. On the West Coast the action is in Japanese yen.

Betting against the buck is what currency speculation amounts to. By buying yen, the investor is counting on the value of the dollar to decrease compared to the yen. In the past year the dollar has dropped 20 percent against the yen: Swiss francs purchased last May are worth 40 percent more than the buyers paid for them.

With the dollar being battered on international money markets, increased currency trading is reported by Deak and Riggs National Bank, the other big trader in Washington.

"We've noticed an accelerated volume in activity on the foreign exchange side," reported James Flippin, head of Riggs' foreign exchange desk.

For Riggs the increase in currency trading has come mostly from multi-national corporations and from importers, who because of the declining value of the dollar find it attractive to pay bills by hedging in the local currency.

But the city's biggest bank also has found people planning summer vacation in Europe are exchanging their money now, anticipating that exchange rates will decline further in the coming weeks.

That's the way many currency speculators start, says Deak's Provini. A tourist goes to Europe and learns nobody wants dollars anywhere. Or a travel agent tells a tourist to change dollars to marks before going to Germany, and to do it as early as possible to make a little money. Or a tourist comes back from Japan with extra yen, cashes them in a few weeks later and finds they're worth more than he paid for them.

From there it's an easy jump to speculative buying income "hard currency" whose value is expected to increase against the dollar.

While Riggs' foreign exchange speculatist says the bank "would tend to discourage" currency speculation, Deak makes a business of it.

"We buy and sell paper," said Provini. "the paper has different colors. Because of supply and demand, some colors are worth more than others."

Currency specualtion has traditionally been an investment vehicle for sophisticated investors, wise in the way of international economics.

"The difference is now we have people coming in who don't know what the Swiss franc is, but they want to buy it," said Provini.

Some buyers, he suggested, are small investors who got burned in the stock market when the go-go days went, but are bored with watching their money sit in some savings and loans earning 7 percent per year.

Deak Washington is one branch of a international financial operation, Deak-Perera. with officers from Guam - where former Marine Corps officer Provini was hired - to Switzerland - where it owns banks in Zurich and Geneva.