PETA said on Monday that SeaWorld is invoking a Securities and Exchange Commission rule to prevent its shareholders from learning about the proposal.
PETA owns a small number of SeaWorld shares.
Under PETA's proposal, SeaWorld would develop coastal sanctuaries where the company's orcas could retire.
PETA says SeaWorld is trying to exclude the PETA proposal by sending out proxy materials on April 17, just two days shy of the one-year anniversary of when SeaWorld went public.
A company is allowed to exclude a shareholder-initiated proposal if the shareholder has held stock for less than a year.
In a statement, SeaWorld says it doesn't comment on shareholder proposals.