It started on Friday - when Azealia told a US radio station that Cardi was an "illiterate, untalented rat" and a "caricature of a black woman".
Billboard Magazine reports that - in a now inaccessible Instagram post - Cardi responded by saying she'll always stay true to herself.
Her Instagram account has since gone offline.
In Friday's interview with The Breakfast Club, Azealia Banks was typically outspoken about her fellow rapper.
She said black female culture peaked two years ago with the Beyonce album Lemonade - but since then things have gone "like Cardi B".
"I'm just talking about this caricature of a black woman that black women themselves would never be able to get away with," she continued.
"Like, if my spelling and grammar was that bad, I'd be cancelled. If Nicki Minaj spelled like that, we would be ragging on her all day."
Fans have since been sharing a screen grab supposedly taken from Cardi's Instagram before it was deleted.
In it, she says she can speak two languages fluently.
"Just because I mix a few words up, forget to use commas or misspell a few words doesn't make me illiterate and doesn't make me stupid," she says.
"And because I laugh a little harder or talk a little louder doesn't make me a caricature!"
She then calls on Azaelia to "pray for [her] own success before [she prays] for the downfall of others".
The two rappers have fallen out before. Back in September Azaelia called Cardi a "poor man's Nicki Minaj".
Newsbeat has contacted Cardi B's management to see if she has any further comment.
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