Robert Samuelson's June 6 op-ed column "Competition: Still the Secret of Economic Success" is a dangerous half truth -- very dangerous as we enter the 1990s with significant global excess capacity in many high-technology, as well as basic, industries.

In supporting competition, Adam Smith was reacting against the monopoly of the medieval guild system; he was not supporting unlimited and unchecked competition as an absolute means to prosperity in and of itself, nor was he supporting the kinds of contemporary zero sum and negative sum competition Mr. Samuelson refers to as "cannibalistic."

Mr. Samuelson's new-found understanding of the "ferocious rivalry" among large Japanese firms is a well known phenomenon that the Japanese themselves refer to as "excess competition," a condition in which prices cover average variable but not average total costs. No industry can survive such a condition in the long run without subsidy or extended private credits, but Japan actually promotes excess competition in high-growth sectors in order to breed a super-efficient core of companies.

In Japan, the inevitable shake out that eventually accompanies such sustained periods of excess competition is a carefully monitored and regulated process, and it is steered in directions that promote the general welfare.

The United States has experienced excess competition both by design and by accident in the past decade. But it has adopted a much more laissez faire attitude toward it, with mounting adverse consequences. Deregulation created a condition of excess competition in many industries. That may have served consumers' interest but was not a sustainable economic process for producers, even very efficient ones in industries like organized trucking.

Competition is still a secret of economic success, but in the regulation of competitive excess, we also may have something to learn from Japan. JAMES A. CLIFTON Rockville