Fewer than one-third of the former Defense Department employes who go to work for defense contractors are reporting their employment as required by law, according to a General Accounting Office study released yesterday.

The study is part of an ongoing investigation by the GAO into the "revolving door" phenomenon, in which senior military officers and Pentagon civilians leave the government for jobs in the defense industry.

Only 18 percent of the Navy and Marine Corps officers, 14 percent of the Army officers and 8 percent of the civilians filed the requisite notice of defense-contractor employment, even though failure to file can result in a $1,000 fine and up to six months in prison.

Air Force officers showed significantly better compliance, with 61 percent reporting. The GAO attributed that to a unique Air Force policy of annually notifying retired officers of the filing stipulations. The overall compliance figure was 29 percent for officers and civilians.

"The 'good old boy' network that constantly promotes Defense Department officials into civilian jobs, where they can make good use of their Pentagon contacts to watch out for the interests of their new employers, continues to thrive right under the nose of the Pentagon bureaucracy," said Sen. William V. Roth Jr. (R-Del.), the Governmental Affairs Committee chairman, who released the study with Rep. John E. Porter (R-Ill.).

A Defense Department spokesman said the Pentagon had no comment on the study.

The GAO quoted Defense Department officials as expressing "surprise at the low level of reporting compliance."

The study points out that the GAO "does not address the question of the extent to which such employment constitutes a potential or apparent conflict of interest."

Legislation under consideration on Capitol Hill would strengthen the reporting requirements and establish administrative sanctions of up to $10,000 against former Pentagon employes who fail to comply.

Federal law currently requires that senior officers and civilians who earn at least $15,000 annually from major defense contractors must report their employment for up to four years after leaving the Pentagon.

Rather than investigate all 58,000 senior military and civilian employes who left the Pentagon from 1980 to 1983, the GAO study used statistical sampling techniques that provided a 95 percent confidence level in the accuracy of the overall filing numbers.