A California district judge on Tuesday dismissed a counterclaim filed by Psystar that accused Apple of violating federal and state antitrust laws.

Psystar argued that Apple's operating system is an independent market with no competitors, but the judge disagreed.

"Psystar has cited no decision lending support to its single-brand product market theory," Judge William Alsup wrote in his decision. Psystar's own claim explains that the Mac OS performs the same functions as other operating systems, he said.

Apple sued Psystar in July for copyright infringement, breach of contract, trademark and trade infringement, and unfair competition after Psystar started selling the Open Computer, which uses a modified version of Apple's OS X.

Psystar filed a counterclaim in August, arguing that a market could exist for third-party Mac clones, if not for the legal and technical barriers that Apple has set up.

Psystar argued that demand would not be affected if Apple increased prices for its OS and said that similarly configured computers cost much less than Apple machines.

"Although the responsiveness of one product to the price of another is at the heart of the market definition analysis, a mere price differential alone does not necessarily signal a distinct market [or] mean that a product is unconstrained by competition," Judge Alsup concluded.

Psystar also cited Apple's ubiquitous advertising campaign as evidence of its market dominance, but the judge concluded just the opposite.

"Those advertising campaigns more plausibly support an inference contrary to that asserted in the counterclaim--vigorous advertising is a sign of competition, not a lack thereof," according to the ruling. "If Mac OS simply had no reasonable substitute, Apple's vigorous advertising would be wasted money."

Last month, Apple and Psystar agreed to enter into a dispute resolution process in an effort to resolve Apple's suit against Psystar. According to court documents, they have until January 31 to negotiate a settlement. If no such agreement is reached, a jury trial is set to begin on November 9, 2009.