By comparison, Linux accounted for 61% share of the Top 500 list in 2005, while the highest ranked Unix system was Hewlett-Packard's HP-UX at nearly 15%. The latest supercomputing list will be out Monday.

Despite the latest numbers, it is important to note that Unix still accounts for a major share of server revenue, and remains an important part of the hardware mix.

Jean Bozman, an analyst at IDC who follows this market, cited a combination of factors in the sharp drop in Unix server sales in the first quarter.

First, she said, the recession delayed sales of Unix servers, which are typically replaced every five to seven years -- longer than most x86 systems. Unix server buyers may also be holding back as Oracle Corp. moves to fully absorb Sun Microsystems Inc. after its deal to buy the key Unix server vendor closed in January.

Further, Bozman noted that Unix server users may be waiting for product upgrades from Hewlett-Packard Co., which recently announced new products in its Integrity line, and IBM, which is due to release Unix servers later this year.