Olympic gymnastics champion Simone Biles is going head-to-head with one of the most vocal opponents of transgender girls and women competing in female sports, former college swimmer Riley Gaines.

Biles — the most decorated Olympic gymnast is history — called the former college athlete “sick” and a “sore loser” in a post on X Friday night.

“You should be uplifting the trans community and perhaps finding a way to make sports inclusive OR creating a new avenue where trans feel safe in sports,” Biles, 28, wrote. “Maybe a transgender category IN ALL sports!!”

“But instead… You bully them,” Biles continued. “One things for sure is no one in sports is safe with you around!!!!!”

As a college swimmer, Gaines tied for fifth place in a competition with then-University of Pennsylvania swimmer Lia Thomas, who is transgender, in 2022.

Since then, Gaines has largely built a career as a pundit in conservative media advocating against trans women competing in female sports, arguing that it is unfair to include them in competition and their inclusion is at times unsafe for other competitors.

Gaines, 25, has also built an enormous following on social media, with over 1.5 million followers on X, where she regularly lambastes and mocks transgender athletes.

On Friday, Gaines posted an image of a Minnesota girls high school baseball team, which supposedly includes a trans player. She appeared to taunt the player, referring to her as a boy, which appears to have elicited Biles’ response.

Gaines almost immediately responded to Biles’ criticism in a series of posts on X.

“This is so disappointing. My take is the least controversial take on the planet,” Gaines wrote. “Simone Biles being a male-apologist at the expense of young girls’ dreams? Didn’t have that on my bingo card.”

The issues of trans girls and women competing in female sports has become a lightning rod issue in the United States in recent years.

In February, President Donald Trump signed an executive order aiming to prohibit transgender women and girls from competing in female sports.

Before that, at least 27 states already had laws, regulations or policies banning transgender students from participating in sports consistent with their gender identities, according to the Movement Advancement Project, an LGBTQ think tank.