UPDATE 1 S.African competition commission probes grocers

* Probes Pick 'n Pay, Shoprite, Woolworths, Massmart, Spar

* Concerned about pricing, buying power

* Will decide whether to refer to Competition Tribunal

* Pick 'n Pay falls 2 pct; food retail index dips

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JOHANNESBURG, June 29 (Reuters) - South Africa's Competition Commission has launched an investigation into major supermarket chains over possible price fixing and other antitrust concerns, hitting some retail stocks.

The Commission said in a statement on Monday it was investigating Pick 'n Pay (PIKJ.J), Shoprite (SHPJ.J), Massmart (MSMJ.J), Woolworths (WHLJ.J) and Spar (SPPJ.J) over concerns about possible anti-competitive behaviour.

Shares in Pick 'n Pay slid 2.2 percent after the news. Massmart dipped 0.6 percent, Woolworths fell 0.24 percent, Spar dropped 1.53 percent and Shoprite rose 0.96 percent.

The JSE food and drug retailers index .JFOOR slipped 0.18 percent and the general retail index .JGERE was flat, compared with a slightly firmer All-share index .

"This complaint initiation now aims to uncover if there are any possible competition concerns arising in the retail space, where consumers would most directly be impacted," Commissioner Shan Ramburuth said in a statement.

The authority will decide after the investigation whether to refer the supermarket chains for hearings to the Competition Tribunal, which has the final say on antitrust issues.

The Commission said its concerns included the concentration of buyer power which made it difficult for small producers to get products on supermarket shelves, and long-term exclusive property lease agreements.

It was also concerned supermarket groups are exchanging price sensitive information, which could hurt competition. (Reporting by Rebecca Harrison)