Bulger, the former leader of the Winter Hill Gang, is charged in a 32-count racketeering indictment that accuses him of participating in 19 murders in the 1970s and `80s. He is also charged with extorting bookmakers, drug dealers and others running illegal businesses.
Martorano is one of three former Bulger loyalists who struck deals with prosecutors and agreed to testify against him. Bulger's former partner, Stephen "The Rifleman" Flemmi, and former Bulger lieutenant Kevin Weeks also are expected to be key prosecution witnesses.
During his opening statement to the jury, Bulger's lawyer, J.W. Carney Jr., raised questions about the credibility of Martorano, Weeks and Flemmi, citing the "extraordinary" plea deals they struck with prosecutors.