Applicant applies.
Respondent responds to applicant.
Applicant can only reply to material in the response, not raise new material.
I've assumed this is for a FCC and the OP has said this is the 'first' hearing. I'm unclear if it is just a directions hearing, in which case there's no panic, or an interim hearing where interim orders will be sought, in which case get a reply in ASAP (eg tonight).
Ok, I'm off to write a response.