DUBAI, United Arab Emirates (AP) — Saudi Arabia's stock market, valued at $585 billion, opened up to direct foreign investment for the first time Monday, as the kingdom seeks an economic boost amid low global oil prices.

The opening of the Tadawul Saudi Stock Exchange allows companies, particularly those that are not in the oil business, to raise money straight from foreign investors, with the goal of expanding businesses, diversifying the economy and creating more jobs for the kingdom's growing population. Before Monday, foreigners only could access the market indirectly, through a local Saudi institution, which was costly and complicated.

The stock exchange's estimated value makes it the biggest in the Middle East. Petrochemical firms make up a fifth of Tadawul, with heavyweights like Saudi Basic Industries Corp. among those listed.

The move comes at a crucial time for Saudi Arabia, whose revenue has suffered from a plunge in oil prices over the past year. That lower revenue could constrain government spending, which in turn would affect the many companies relying on government projects. The kingdom has been drawing from its robust foreign reserves to maintain spending.

An influx of foreign money could "help to plug some of the external shortfall and slow the pace at which Saudi Arabia is drawing down its reserves," says the London-based analysis firm Capital Economics.

The firm says Saudi Arabia has been traditionally cautious about foreign influence in its political and economic affairs. Its decision to open its stock market could be seen as part of a broader liberalization effort in the kingdom's economy. The socially and religiously ultraconservative country is already awash in some of the world's biggest brands and many multinational companies have their factories and facilities there.

However, foreign investors say they are taking a cautious approach and warn not to expect an immediate rush of foreign investment into the Middle East's biggest market.

"In the immediate to short term, the money flow will be gradual," says Sachin Mohindra, Gulf portfolio manager for Invest AD.

In fact, the index was slightly down Monday, closing just shy of a percentage point below Sunday's closing mark. Tadawul, like other regional indexes, follows the traditional Middle Eastern work week with trading taking place Sunday to Thursday.

John Sfakiankis, Middle East director based in Riyadh at emerging markets investment firm Ashmore Group, says he too does not expect a big influx of foreign investment right away. He expects a gradual flow over the next few years of $20 billion to $25 billion.

One reason for the cautious approach: When local investors anticipated the opening of the market, they bid up stock prices, leaving them overvalued in the opinion of fund managers.

Additionally, there are regulations in place for foreign investors and it remains unclear how many foreign institutions have been granted licenses. Only financial institutions with $5 billion or more of assets under management that have been in operation for five or more years are eligible to invest, thought the regulator says it could make exceptions.

Other regulations are that qualified foreign investors cannot own more than 5 percent of the shares of any company. These investors as a whole cannot own more than 20 percent of shares in the roughly 165 listed companies.

There are five companies that will be off limits to foreign investors. Most are in construction in the Muslim holy cities of Mecca and Medina, which are closed to non-Muslims.

The kingdom's stock market regulator, the Capital Markets Authority, says the decision to open the market to direct foreign investment is aimed at supporting increased participation of institutional investors and reducing the role of smaller investors.

According to Tadawul, Saudi individuals make up 34.4 percent of stock market ownership, but account for nearly 90 percent of trading activity. That has exposed the market to volatility.

Tadawul CEO Adel al-Ghamdi said at an economic conference in Riyadh last month that the framework was specifically customized to attract "sophisticated, longer-term" investors that take an active role in shaping and questioning the direction of the companies they are investing in, which would lead to higher standards of corporate governance.

"The point of the framework was never about increasing foreign investment flow into the kingdom. It was always about trying to bring foreign investors to enhance our practices and our market infrastructure," al-Ghamdi said.

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