The jurors were identified in court only by number on the ninth day of jury selection, NBC News reported.
The jurors and four alternates will be sequestered during the trial, expected to last two to four weeks.
Zimmerman, a 29-year-old Hispanic American and one-time Neighborhood Watch volunteer, has pleaded not guilty to the second-degree murder charge, saying he shot the 17-year-old Martin in self-defense after the unarmed black teen attacked him during a confrontation in Sanford, Fla., Feb. 26, 2012.
Circuit Judge Debra S. Nelson is expected to rule on whether state experts can testify about a 911 recording on which screams and the gunshot that killed Martin can be heard.
Two state experts said the screams did not come from Zimmerman. The defendant disputes the experts' opinion, saying he was fighting for his life and calling for help.
The Orlando (Fla.) Sentinel said Zimmerman's parents, Robert and Gladys, were in court Wednesday for the first time since the trial began. The parents didn't talk to media, but the defendant's brother, Robert Zimmerman Jr., said in a statement he spent the day caring for his grandmother, who has Alzheimer's disease, so his parents could attend the hearing, despite "numerous death threats."
The New York Times said Nelson is considering a defense request to maintain jurors' anonymity for a six-month cooling-off period after a verdict is reached.