As well as US unit sales falling, Apple's Mac revenue share is also falling.
Retail research firm NPD Group told the Wall Street Journal that, in addition to a 6% year-on-year drop in January Mac unit sales, the value of sales through US retail channels fell 11% in January.
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Apple's market share in dollars declined to 13.7% from 16.4% in January 2008, NPD added.
Apple has not responded to the widely reported estimated figures from NPD, which says consumers are favouring its cheaper models in the face of the credit crunch.
The predicted fall in Mac unit sales would be the first year-on-year quarterly fall since 2003. The quarter under scrutiny ends in March.