BRAD LEITHAUSER, whose Hundreds of Fireflies (Knopf) was one of the final contenders in poetry for a 1983 National Book Critics Circle Award, was recently visiting this country from Japan where he's lived since graduating from law school. ("I wanted to do something weird," Leithauser explains. The two "weirdest" offers for legal work he had at that time involved going to Guam or Kyoto, and, after consultation with his wife, Mary Jo Salter, also a poet, he chose the latter.) One thing he did not expect to happen during his stopover in the United States, while he did some job-hunting, was to find himself the recipient of one of the unsolicited MacArthur Foundation awards. His is $30,400 a year for the next five years, with no strings attached.

It's a little odd to be a serious (successful) poet and a lawyer, but not, when you come to think of it, all that strange. Louis Auchincloss, for example, is a lawyer and a novelist; Wallace Stevens worked as an insurance executive; L.E. Sissman was an advertising man; T.S. Eliot started off in a bank. But to the anonymous committee which bestows the now-coveted MacArthur gifts, Leithauser is special enough. With one-third of a new book of poems completed, he's also at work on a novel. However, he's not sure what actual effect the MacArthur money is going to have on his plans. One thing: they give the cash out on an age basis, with the younger winners getting less, and the 29-year-old Leithauser wouldn't be human if he didn't admit, "It's the first time I've ever wanted to be older than I am."