WHENEVER YOU SEE a particularly memorable wildlife photograph, the odds are that the credit line says "Photo by Leonard Lee Rue III."

Rue, 57, has published more pictures of wildlife than any other American, and perhaps more than anyone else in the world. He's done more than a thousand magazine covers, with as many as five of them hitting the stands the same day. He has more than a million slides and negatives on file, and his darkroom in Blairstown, New Jersey, produces about 100,000 black-and-white prints a year.

Rue's the delight and despair of many of us who also have tried to capture wild creatures on film. The delight comes from looking at his work and the despair from trying to match it. Rue's photos are sharp, rich-textured and strongly composed; the animals are alert, poised, pulsing with life and purpose, and are shown in intimate association with each other and in clear relation to their habitats.

Anyone who's tried it knows it's tough enough just to get any kind of picture of a wild animal, much less the sort of crisp classics Rue produces, some of which have been in print almost constantly for decades. But then he does it for a living, he's out there shooting all the time and he probably throws away a hundred shots for every "keeper," right?

Wrong. "I get out maybe four months a year," Rue says. "The rest of the time I spend researching, writing, lecturing and trying to keep half-a-dozen businesses going." And as for wasted film, no: "On a typical day in the field I shoot about 20 rolls of 36-exposure 35mm film, and I come back with an average of 30 salable shots per roll."

The serious nature photographer will find a couple of hard truths in those statements. The first is that, with a batting average of over .800, Rue's in another realm of expertise from most of us mortals. But even more daunting is the fact that although he's the most successful wildlife photographer, Rue has to spend two-thirds of his time doing something else.

"It was many, many hard and hungry years before I even began to break even," he says. "And there's never been a time when I could just take pictures and not worry about anything else, because you have to hustle all the time. I work seven days a week, year in and year out. If I didn't love it, it would kill me."

Now he's Leonard Rue Enterprises, a family of firms he operates with his son, Leonard Lee Rue IV, whose photo credit has been simplified to "Len Rue Jr." because editors kept screwing it up. When you see a great nature photograph that Len III didn't take, Len IV probably did. Beside being able to supply color or black-and-white photos of virtually any mammal, bird or reptile in the world, the firm stands ready to teach you how to take your own wildlife pictures and to sell you such aids as masking scents, blinds, camouflage clothing and specialized camera gear Rue designed or selected, plus gorgeous wall posters to remind you of what you're after. Also, somehow, Rue manages to find time to deliver a couple of hundred lectures every year.

Oh, and then there are the wildlife books, of which high-school dropout Rue has written 19, the latest being the just-published How I Photograph Wildlife and Nature (Norton, 287 pp., $19.95). Like all of Rue's books it is gracefully written and wonderfully illustrated, solid in its facts, clear in its explanations and deficient in nonsense.

Nineteen books would constitute a career for most men, especially when they include such works as the recent and definitive The Deer of North America, for which Rue's research included reading everything available on the subject, much of which he shows to be myth, sloppy science and wishful thinking.

Rue's mastery of the disparate and demanding disciplines of photography, writing and public speaking all are the fruits of his true vocation: He is a lifelong, fulltime naturalist. But as he says without wasting any of our time on false humility, "I am best known for the superb animal portraits I take."

And as the book spells out, Rue gets those superb portraits because he studies the animals, knows his cameras and film, and is aware of his own strengths and limitations. Alas, the secrets of great wildlife photography are those same damned old eternal verities: study, practice, work work work.

Learning how Rue does it is fascinating, and the pictures are of course splendid. If it's not in your bookstore, it's available from Leonard Rue Enterprises, Blairstown, New Jersey 07825.