NEW YORK (AP) _ For the first time since Forbes began compiling an annual list of the richest entertainers four years ago, the No. 1 position goes to neither Michael Jackson nor Bill Cosby.

Teeny bop sensation New Kids on the Block has taken over as the top-paid act, Forbes says in its Sept. 30 issues, released Sunday. The musical group's projected earnings for 1990 and 1991 will total $115 million, it says.

Cosby, who collects millions every month on syndicated reruns of the ''Cosby Show'' alone, comes in a close second. His 1990-91 earnings will total about $113 million, the magazine says.

Third and fouth place, respectively, were taken by Oprah Winfrey and Madonna. Winfrey will earn an estimated $80 million in 1990-1991, while Madonna's raunchy lyrics and stage act will give her $63 million.

Jackson ranks fifth this year, down from second last year. His estimated 1990-91 earnings are $60 million.

The estimated gross incomes are based on two-year projections because of the lag time between an entertainer's work and the compensation for it.

Forbes said its figures were calculated from talks with industry executives, lawyers, agents and managers. It said earnings figures for recording artists were based on royalty payments from record and video sales and estimated pretax gross for concert tours.

Rounding out the top 10 were Kevin Costner, a newcomer to the list, with earnings of $59 million; Johnny Carson, $55 million; The Rolling Stones, $55 million; Peanuts creator Charles M. Schulz, $51 million; and director-producer Steven Spielberg, $50 million.

Other names new to the top 40 list included rapper M.C. Hammer, at $33 million, ranked 19th; Arsenio Hall, with an estimated $23 million, ranked 35th, author Tom Clancy, at $20 million, ranked 36th, and rapper Vanilla Ice, at $18 million, was No. 40.