Compare two very large tables

Ok,
So im looking to compare two very large tables.

TableA + TableB

TableA is the table used by the site, and TableB is imported data.

I want to compare the two, and if a row is found in TableA, that matches a row in TableB, I want to delete it (from TableA)

Anyone got any ideas?

Thanks Calum

delete A.* from A join B on A.key = B.key


something like that.

But it's important that the two keys shown in that query are each an index on their respective tables. Else the query will run really slowly.