Long before there was an Environmental Protection Agency, Gershon Fishbein made a living telling people what they needed to know about environmental issues.

His Environmental Health Letter marked its 20th anniversary last month, the oldest in the environmental business and the flagship of a one-man fleet of newsletters that has prospered in a field littered with failed publications.

Now there are hundreds of newsletters, specializing in technical minutiae and behind-the-scenes reporting within every industry, scientific and governmental subdivision. The Newsletter Clearinghouse Directory lists 2,400 newsletters in 150 categories, and about 400 of them are based in Washington, watching federal agencies at very close range. At least 50 specialize in environmental news, and customers include their targets: EPA, for instance, subscribes to 30 newsletters, according to an agency official.

"Somebody once called us the 4th-and-a-half estate," Fishbein said in a recent interview. Little known outside their fields, the newsletters rarely spark Hill hearings or win big prizes for their investigative work, even though major newspapers often get tips or ideas for whole stories from them.

The majority are run by a few people who work hard for not too much money. If the newsletters survive the first two years, they tend to grow into multi-publication operations, according to Fred Goss, executive director of the Newsletter Publishers Association.

"Basically we're all inside-dope addicts," Fishbein said.

McGraw Hill's Washington Report on Medicine and Health launched newslettering in that field in 1947. Fishbein's was next, and then came the Business Publisher's Air/Water Pollution Report in 1963 and the Washington Drug Letter in 1969.

"The big explosion was in the mid-60s with the Great Society and the growing demand for specialized information," Goss said. The Bureau of National Affairs alone now has six environment-related publications.

Fishbein started his newsletter at the age of 40, when he decided that he didn't want to work for some one else anymore. A former reporter and editor at The Washington Post, Associated Press and the New York Herald Tribune, he was running the Washington office of the Medical Tribune, a magazine for doctors, when Congress began reorganizing the Public Health Service.

"It was old former Democratic representative John Fogarty of Rhode Island who suggested it," Fishbein recalled. The introductory issue accurately predicted that a new Bureau of Environmental Health would soon be set up within the Public Health Service.

He sent the issue to a list of 2,000 potential subscribers, offering it for $50 a year, and got 125 buyers, a good return. Now EPA has 10,000 employes and Fishbein's newsletter, still bimonthly, sells for $110 a year. His empire now includes the Occupational Safety and Health Newsletter and the Genetic Engineering Letter.