The Pentagon suspended C3 Inc., a Reston computer systems company, from bidding on defense contracts yesterday because of false statements allegedly made by the company in connection with an Army contract.

Virtually all of C3's contracts are with the government, and most of those are from the Pentagon. Its stock, which is traded over the counter, plunged by $6.75 to $9.75 bid yesterday.

Company officials said in a statement that they would "vigorously contest" the suspension. C3 executives, including Chairman and President John G. Ballenger, and the company's lawyers could not be reached yesterday for additional comment.

The ban on the company -- which does not affect its existing Pentagon contracts -- is indefinite, according to an Army spokesman. C3 will be able to appeal the ban to the Army's judge advocate general's office. Although the suspension covers bids by C3 for contracts with all branches of the military, the Army is handling the case because one of its contracts is the one in dispute.

"They were suspected of making false statements and claims" on an Army contract, said the Army spokesman. He would not give any other details, but he said only one of C3's government projects was being disputed.

Such suspensions of bidders is unusual, the Army spokesman said, with the last occurring earlier this year.

C3, which had $48.6 million in revenues last year and employs about 200 people, supplies computer systems and software to all of the services. Its most recent large contract with the Army was a $45.7 million pact, awarded in June, to supply video microprocessors to be used in all Army recruiting offices. It is not known whether this is the disputed contract.

C3 -- the name comes from "computers, communication and control"--was founded in 1968 by a group of former executives of an RCA Corp. unit that dealt with government contracts.

Earlier this year, the company was disqualified from bidding for a $1 billion Navy contract for a tactical computer program. C3 said the disqualification was based on the Navy's analysis of the company's proposal and a demonstration of the firm's hardware and software. Its stock dropped sharply at that time, too.

C3 has registered healthy increases in sales and earnings over the past few years, and earlier this year earned a listing on Inc. magazine's compilation of the nation's 100 fastest growing companies. Sales in the fiscal year ended March 31 were up nearly 50 percent, and earnings rose 68 percent to $8.4 million (98 cents a share).

Sidney Wachtel, head of the local investment firm that bears his name and one of the financial backers of C3 when it was founded, said yesterday he expects the ban to be short-lived.

He said that the company probably could weather even a prolonged ban on bidding because of its backlog of government contract work, and said he was continuing to buy C3 stock.