Both locations of the Harvard Bookstore were closed briefly yesterday afternoon after the company's main office on Plympton Street received what appears to have been a prank phone call from a man threatening to set off a bomb in one of the stores.

Paula Bright, comptroller of Harvard Bookstore, said yesterday she notified Cambridge police and firemen and warned the branch offices after she had received the threat. The caller did not identify himself and did not mention a reason for the threat.

'The caller didn't really say anything other than that a bomb was going to go off at the store." Bright said. "I couldn't tell whether it was a serious threat or just a prank," she added.

Bright said the caller did not say which store he had placed the bomb in.

By the time police arrived at the stores, both located on Mass Ave. salesmen had already evacuated all customers. Within minutes. Cambridge fire fighters were searching the stores for explosives, but they were unable to find any. After a search lasting nearly an hour, the firemen determined that the threat was false and the stores were re-opened.