Vietnam's State Securities Commission is taking measures to make the stock market more transparent and to guard against irresponsible speculation. The market has risen rapidly in recent weeks, and authorities are concerned that a bubble may be forming. In Hanoi, Matt Steinglass has more.

The VNIndex closed at 1028 points on Monday, putting Vietnam's stock market up more than 35 percent since the start of the year.

Vietnam's State Securities Commission has announced new measures to cool the market. To slow foreign investment, it will maintain, for now, a cap limiting foreign ownership to 49 percent of a company's shares.

And also Vietnam's State Bank says banks must limit loans for purchasing stocks to 20 percent of their total capital. Borrowers will be required to mortgage real assets for loans used to purchase stocks.

Vietnamese authorities are concerned that investors, rather than making informed decisions, are simply following the crowd. Pham Hong Son is the vice administrator in chief of the State Securities Commission.

Son says Vietnamese investors have what he calls a "buffalo-herd psychology," because the market is dominated by individual investors looking for short-term gains. Developed markets like in the United States, he says, are more influenced by the decisions of large fund managers, who have longer-term strategies.

The Securities Commission also moved to investigate complaints that stock brokers unfairly delay trades and refuse to release information about companies.

Nguyen Hang Hoai, general secretary of the Vietnam Association of Financial Investors, says the problem is that securities companies have a monopoly on financial information.

Hoai says Vietnamese listed companies send their financial reports only to stock exchange staff, who then take several days to make them public. In the meantime, the stock exchange staff may use the information for what Hoai calls "planted spy transactions" - effectively insider trading.

The government will demand that companies make their 2006 financial reports public more rapidly. It says it will investigate traders who may abuse information, and fine violators.

The government also will require foreign investment companies to register with the Securities Commission.

The government hopes these measures will bring transparency and stability to the stock market. Market analysts say financial reporting by many Vietnamese companies remains opaque and misleading.

Government officials say that Vietnam must clean up its securities markets to avoid the risk of a correction, and keep the stock market going up.