• by Joe Mandese  @mp_joemandese, July 3, 2009

The economic crisis is taking a toll on some of the world's largest agency holding companies. Analysts at Citigroup downgraded shares of WPP Group, the world's biggest buyer of media, to "sell" from a "hold" position, citing especially weak organic revenue trends during the second of 2009, and weak visibility for the foreseeable future.

The Cazenove Group, meanwhile, slashed its rating for another big media services holding company, Aegis Group, to "in-line" from "outperform" due to a recent spike in the price of Aegis' shares, coupled with weakening media marketplace trends, and weak interim results.

Aegis' stock is trading around $1.40 a share, about double its 52-week low of 73 cents a share.

Shares of WPP's stock, meanwhile, are down nearly 7% following Citigroup's report, and are at around $31.59, down from its 52-week high of $49.86.

The Citigroup analysts cited WPP's dependence on a "largely fee-based model," and alluded that agency stocks in general would "lag" any economy recovery.