Taken from an article in Research News. I've lost the date. It must have been about 1989, because that's when the event occurred.

The new 65,087-digit prime number, 391581 × 2216193 - 1, was found by a team of computer scientists from Amdahl Corporation. It was notable not only because it was a new record, but also because it was not a Mersenne prime. Almost all "largest prime number" records lately are set by Mersenne primes. For example, all four primes larger than this one that have been found to date (Spring 1997); see this cool prime number page for more details.