Prosecutors allege Bonds lied when he told a federal grand jury in 2003 that he never knowingly used performance-enhancing drugs.
Bell met Bonds in 1994 and testified that from 1999 to 2001, "he was just increasingly aggressive, irritable, agitated, very impatient."
In testimony similar to that of former Bonds business partner Steve Hoskins last week, she said that in at least 2 years at spring training, she saw Bonds and personal trainer Greg Anderson "go into a bedroom off the kitchen and close and lock the door."
She said Anderson "would always have a little satchel with him." She saw those scenes played out multiple times.
Prosecutors claim Anderson, who has been jailed for refusing to testify, repeatedly injected Bonds with performance-enhancing drugs.
Bell answered 72 minutes of prosecution questions and was pressured during 4 hours, 15 minutes of questioning from the defense, who tried to portray her as a gold digger, a scorned former lover, a liar and the instigator of a mortgage fraud scheme.
Defense lawyer Cristina Arguedas brought up an interview Bell gave Playboy and a television appearance on Geraldo Rivera.
"You have taken many opportunities to disparage Barry Bonds ... in the most vulgar ways possible?" Arguedas said in a question that was more a statement.
"Did you go on Howard Stern's radio show?" Arguedas continued. "Does he do anything that isn't vulgar?"
When Arguedas repeated: "Did you say vulgar things about Barry Bonds?" Bell answered: "Please refresh my memory."
With that, Arguedas took a break to talk with Allen Ruby, Bonds' lead lawyer. After a few moments, Arguedas told the court: "We're going to decline that opportunity to go into the gutter. No more questions."
At the start of the day, Giants equipment manager Mike Murphy testified that Bonds' hat size increased from 7 1/4 to 7 3/8 in 2002.
Former Giants head athletic trainer Stan Conte is to testify today along with former AL MVP Jason Giambi, brother Jeremy Giambi and Randy Velarde, other players linked to the Bay Area Laboratory Co-Operative, which ran a steroids distribution ring.
While there were empty seats in the court room last week, the wood benches were filled for Bell's testimony and about a dozen people waited on line outside for one of the approximately 50 seats available to the public.
Bell testified Bonds revealed his steroids use to her only once, between 1999 and 2000 at her apartment.
"He had an injury on his elbow and it was a big lump on his elbow," she said. "It looked really awful, and he said it was because of steroids . . . somehow it caused the muscle and the tendons to grow faster than the joint itself could handle."
Bonds had left elbow surgery on April 20, 1999, and was on the disabled list until June 9.
Under questioning from Assistant U.S. Attorney Jeffrey D. Nedrow, Bell said Bonds told her "he didn't shoot it up every day like body builders did."
"That's how they were getting ahead, that's how they were achieving, by using steroids," she quoted Bonds as saying. She went on to say this was the period "when Mark McGwire was breaking records."
Bonds alternately watched Bell on the stand, scribbled notes and whispered to one of his defense attorneys, Allen Ruby.
Arguedas spent most of the day trying to portray Bell as a jilted woman who had broken off her previous relationship on the day she was to be married.
When Bonds told her in 1998 that he was going to marry Watson, Bell said the player told her "you can come see me on road trips." Bell testified that after Bonds married, he told her there were "girlfriend cities and wife cities" and that she wasn't allowed to travel with him to New York, Montreal and Atlanta. Bell said she went instead to San Diego, Houston and Miami.
Arguedas quizzed Bell about an e-mail she sent to Bonds' website in April 2004, almost a year after their breakup on May 23, 2003. Bell said she listed all the women she knew that Bonds was sleeping with: a model in New York, another woman in Las Vegas and "the stripper from Phoenix."
"This is the guy who you described as having penile dysfunction," Arguedas said. "That's a lot of action." Bonds covered his mouth in an apparent attempt to suppress a grin.
"I don't know what he was doing with them," Bell responded. "I can only imagine."
* Authorities in Port Charlotte, Fla., are investigating a burglary at a home being rented by Tampa Bay players Evan Longoria, David Price and Reid Brignac during spring training. According to the Charlotte County Sheriff's Office, the items stolen Saturday were valued at nearly $56,000 and included jewelry, cash, computer equipment and electronics.
An AK-47 rifle, which the sheriff's office said was legal and belonged to Longoria, was also taken from the home. "It's a personal item," he said.
* New York Mets outfielder Carlos Beltran (knee tendinitis) is scheduled to play today in his first major league spring-training game since March 6.