Douglas R. casey is a tall, robust fellow of 34 who lives on a nice street in Georgetown. He looks like an international investment counselor with an interest in karate, which he is.

What Casey counsels, however, is economic doomsday as a right-around-the corner likelihood that, far from being bad news, can make you rich. Whether it comes or not, the presumption of the apocalypse has already made him a wealthy man.

His book, "Crisis Investing," subtitled "Opportunities and Profits in the Coming Great Depression." is currently No. 1 on The New York Times best-seller list, where it first appeared last September, and has so far sold just over 500,000 copies in hardback. Any day now, he expects to sell a follow-up book, for investors with no capital, for about $1 million.

Maybe it's cause, or maybe effect, but Casey's best seller transforms its author into an instant pundit, if not philosopher-king, of libertarian popular economics.

What Casey is selling is not just investment advice, but a combat-ready world view that a first appears to be a combination of Ian Fleming, Ayn Rand and Godzilla. Nor does he go out of his way to dispel the image.

"There are always business opportunities when the blood is running in the streets," Casey said at lunch recently. "I like to look for politically creative environments -- places where a government has done stupid things that I can capitalize on."

In Peru, for example, there is a proposal for an arms factory which Casey thinks bears looking into. "It's a start-up venture," he said. "You might be able to buy into it. "Why? Because the '80s are going to be a time of wars, turbulence, rumors and rumors of war. This factory will manufacture the finest assult rifle in the world, which a friend of mine happens to own the rights to. It's a derivation of the M-16 called the PR-19. Of course, I don't recommend this investment for everybody."

Casey has also been looking at real estate in Zimbabwe. In 1979, he points out, a house in downtown Salisbury -- with four bedrooms, swimming pool, tennis court, marble floors and an acre of land -- was going for $25,000. A great buy. Unfortunately, now is the time to sell, since civil war might erupt again. "But the bargains will be back again," he said.

Casey, as a matter of fact, sees far-reaching opportunity in Africa, a kind of economic last frontier of goods and services.

"I'm co-sponsoring a sort of 'life style' show in Soweto," he said. "That ought to please the liberals, though I couldn't care less. What's happening is that the living standard of blacks was so low before that it's now rising at the rate of 25 percent a year. But those people don't have anything to buy -- no theaters, no restaurants, no bars, no stores, no nothing. We'll take a large area, fence it in and display the wares of home-improvement companies, clothing retailers, employment agencies, consumer electronics suppliers, people like that. The retailers from Johannesburg will have to pay through the nose to get in, but they'll want to."

To Casey, the list of governments which have done things all wrong definitely includes the United States.

"We're now bearing the consequences of 50 years of stupidity in this country," he said yesterday. "President Reagan's heart is in the right place, but he's just not radical enough to make any difference. The problem is that when the depresson does come, under Reagan, people may turn back to the liberals. I would much prefer that the economic collapse come under an avowed socialist regime, so you could really see the cause and effect."

The depression that Casey is talking about is not wearing any new clothes; it's the same old Beast from Beneath the Sea of Big Government, spawned of high taxes, federal deficits, inflation, wage-and-price controls, import quotas and regulation of all kinds.

Casey see it all in black and white.

"There really are good guys and bad guys in this world," Casey said. "The good guys are those who believe in free enterprises. The bad guys are the ones who are out to control other people's lives. The fact is that in the end, everybody is responsible for himself. I despise altrusits, because they create hypocrisy and guilt."

Hard-nosed, all right.

"I don't create reality," he says. "I only mirror it. The greatest profits are always made in times of crisis. People who follow my advice will be a new class of millionaire. If you can get your hands on $10,000 now, you can be wealthy by the end ot the decade."

As an investment counselor, he is willing to go out on a limb and tell you precisely how to do it. And he agrues his philosophy -- as he will on a panel to be held March 23 at Lisner Auditorium.

"One thing you never do is follow the herd instinct," he said. "The herd is always wrong, always getting in on something when it looks attractive. But the best investments never look attractive.

"Most people just have the whole thing bassackwards," he said marveling.

"Take the utility companies. Right now utility stocks are selling for half of what they're worth, partly because they have huge debts and because the cleanest, best fuel -- nuclear -- has been slowed way down. But they're an excellent opportunity, because Reagan is going to free up nuclear power. And because inflation is going to wipe out the indebtedness of the plants.

"So utilities look good. But you have to be smart. People are going to cut back on their use of power in the years to come. So what you want is a utility that hasn't just expanded, and doesn't have all that debt. A company that can buy power from other plants is what you want."

Before rushing off to buy into Baltimore Gas & Electric (or Boston Edison or Caroline P&L or Conn Ed or some of the others recommended in "Investing in Crisis," Casey's newsletter), it might be inquired just how successful Casey himself has been.

As it happened, "Crisis Investing" was not an instant hit. It was first published by '76 Press, a California firm, after which a mutual friend brought it to the attention of Robert J. Ringer, a contemporary Horatio Alger whose "Winning Through Intimidation" was the text of the school of hard knocks for the last decade. Ringer, who led Casey to Harper & Row, retains a precentage of the book.

"I never disclose my own financial position," he said, contentedly. "Time magazine misquoted me when they called me a 'self-described millionaire.' But let's say that my personal finances are fine, independent of the books. I sold all my gold last year, at about $650."

Casey, however, see himself less as a millionaire-type than as "an international adventurer who's become an investment counselor." His family has been in the Washington areas since 1840, he said, and he is a graduate of Georgetown University with a degree in International Fund Management.

"Personally, I have a low marginal disability for money," he said. "For me it's just something to go after. I'm really an esthetic type of person. I don't weigh myself down with acquisitions and I like to keep a suitcase packed -- I like to be air-mobile at all times."

In between international adventures -- he's off soon for three weeks in the Far East, followed by three weeks in Australia and New Zealand, 10 days in Switzerland and then a week in Costa Rica -- he pursues the reading of medieval history and science journals. He has trouble pursuing everything else he'd like to, he said.

"I took up sky diving for a while, and made 65 jumps, but there's not much time now. Back in the '60s I campaigned a Shelby Cobra on the drag strips. [His car had a 427-cubic-inch engine; his best time was 115 mph in 11.95 seconds.] and I've been studying karate for the past seven years. The martial arts demand tremendous self-discipline, which also happens to be what investing is all about."

In other words, Casey is not getting married next week, or even the week after. "All the traveling I do, well, you do meet a lot of people on the road You're kind of an exotic being to them. But to have a lady friend in your home town -- well, you have to take her to the movies once in a while. And I'm never here."

After the $1-million book, he owes Dow Jones a volume that will probably be called "The Speculator's Guide," and will discuss fortunes in strategic metals among others.

Oh yes, and last summer a new book occurred to him.

"I was at martial arts camp in Pennsylvania," Casey said, "and there was nothing to do between the sessions. So I started work on a novel It's about an investment counselor Everybody has to write a novel, right?"