Roman Jackiw will deliver the 1998 Dirac Medal Lecture on 26 March 1999 at 11:30 in the Main Lecture Hall on the ground level of ICTP's Main Building. Jackiw, who has held the Jerrold Zacharias chair in the Department of Physics at the Massachusetts Institute of Technology (MIT) for 30 years, will speak on the "Effects of Dirac's Negative Sea on Quantum Numbers." In addition to his long-time position at MIT, Jackiw has been a teacher and researcher at Harvard University, Rockefeller University, the University of California at Los Angeles and at Santa Barbara, and Columbia University. He is the author of six books and a member of the American Academy of Arts and Sciences, American Physical Society and U.S. National Academy of Sciences. The Dirac Medal, which has been awarded annually by ICTP since 1985, is named in honor of the late Paul A.M. Dirac, one of the 20th century's most distinguished physicists and a staunch friend of the Centre. Princeton University's Stephen Adler, who shared the 1998 prize with Jackiw, will deliver his lecture at a later date.