Knox and Italian co-defendant Raffaele Sollecito, her former boyfriend, are being tried on charges of murder and sexual violence in the 2007 killing in Perugia. Both deny wrongdoing.
Mellas took the stand for two hours on Friday, testifying that there were no problems between her daughter and Kercher.
"They got along great," Mellas told the eight-member jury, speaking in a soft, unemotional voice. "She told me about the fun things she and Meredith did," she said, without elaborating.
Last week, Knox testified she was shocked by the death of Kercher, whom she considered a friend. This contrasted with previous testimony by other witnesses that Kercher had complained about Knox's bathroom habits and had expressed surprise at her apparent promiscuity.
Mellas also testified about three phone calls she received from Knox on Nov. 2, 2007, the morning Kercher was found dead in her bedroom with stab wounds to the neck.
"In her first call, she said she thought somebody was in the house," Mellas said.
Last week, Knox recalled going home that morning to find the front door open. She said she took a shower and saw blood in one of the apartment's bathrooms.
The second and third calls were made after Kercher's body was discovered, Mellas said.
"She was very upset, it was disturbing," Mellas said.
Lawyers and prosecutors contended in court that, according to conversations secretly recorded during a visit by Mellas to her jailed daughter, Mellas indicated that Knox had already spoken during her first call home about a foot having been seen in Kercher's bedroom - before the body was discovered.
Kercher's body was found half naked, partially covered by a duvet, with a foot sticking out. Knox testified last week that she heard that "a foot" had been found in Kercher's bedroom when police broke in.
Mellas said Friday that the detail emerged during the second call and that she did not remember that part of the conversation in jail.
Speaking in English through an interpreter, Mellas, who works as a teacher in Seattle, testified Knox never thought of going back to the United States before she was arrested, shortly after the slaying.
"She insisted on staying here," to help authorities and continue her studies, Mellas said.
Mellas and her daughter exchanged looks during the mother's testimony.
Knox's father, Curt Knox, told the CBS "Early Show" from Seattle that his ex-wife's testimony would strengthen his daughter's case. He said the telephone conversations would help understand "the shock that Amanda had after learning that a body was found."
Knox and Sollecito have been jailed for over a year and a half. They could face Italy's stiffest punishment, life imprisonment, if convicted of murder.
Also on Friday, Sollecito's father, Francesco, testified that his son was never violent and would not "hurt a fly."
Prosecutors say that a kitchen knife found at Sollecito's apartment is compatible with Kercher's wounds. The knife had the victim's DNA on the blade and Knox's on the handle.
Prosecutors believe Knox, Sollecito and a third person already convicted in a separate trial went to Kercher's home the night of the murder and killed the 21-year-old in what began as a sex game.
Sollecito, 25, has said he was at his own apartment the entire night of Nov. 1. He said he does not remember if Knox spent the whole night with him or just part of it.
Knox, 21, said she spent the night at Sollecito's apartment and returned home the following morning.
The third suspect, Ivory Coast national Rudy Hermann Guede, also denied wrongdoing but was convicted of murder last year and sentenced to 30 years in prison.
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