The fifth Harry Potter book may not be out until June, but JK Rowling has already started writing the next one, although it doesn't have a title yet.
She revealed this in a letter sent to a fan who'd written to her, according to Potter website Mugglenet.
She says that she hasn't worked out what the title will be - but she says it's great fun!
She also revealed that with her baby due soon she won't have as much time to write.
JK also talks briefly about why The Order of the Phoenix is so long, and how much she enjoys letters from fans.