As a member of the Native Tongue rappers, MC Mos Def made a name for himself with guest spots on De La Soul's Stakes Is High and a Bush Babies single. But it's with current partner Talib Kweli that Mos Def has made an indelible impact on '90s hip hop. The duo's premiere album, Black Star, reads like a 13-song manual on the art of lyrical flow. Def and Kweli trade rhymes like friendly shadowboxers moving in time, although they hurl words with deadly accuracy on tracks like "Definition": Mos Def raps over remixed beats from Boogie Down Productions' "P Is Still Free" in an East Coast ragga style, until Kweli matches Def's rolling delivery to create the best-harmonized chorus of any rap song to date. The pair's unique delivery distinguishes its tracks, and its down-to-earth message is refreshing. At a time when most commercial rappers front riches in videos full of rented cars, Def and Kweli speak introspectively about the state of hip hop as a movement. Songs such as "Astronomy (8th Light)," "Hater Players," and "Thieves in Night" describe the dichotomy between poetry and big business. Black Star's upcoming show at the Justice League is a good place to dust out the cranial cobwebs left by too many rewarmed beats and stale rhymes, and witness one of hip hop's most gifted duos. They play Saturday and Sundayat 9 p.m. at the Justice League, 628 Divisadero (at Hayes), S.F. Admission is $15; call 289-2038.