The Thai stock market regulator said on Saturday that brokers are required to report buying orders from individual clients for 13 shares, including those in firms owned indirectly by the family of Prime Minister Thaksin Shinawatra.
The Securities and Exchange Commission (SEC) said on its Web site at www.sec.or.th that the stocks included television operator ITV, state-owned Thai Military Bank and Thailand's fourth largest bank, Siam Commercial Bank.
ITV is 43 percent owned by Shin Corp, Thailand's largest telecoms group in which Thaksin's son, daughter and relatives have a large holding.
Thai Military Bank is 51 percent owned by the Thai government with Thaksin's son Phanthongtae also a major shareholder.
The SEC said trades in the 13 stocks, among the 50 most actively traded in terms of average turnover for the week ended February 5, must be reported because they had price to earnings ratios of more than 100 times or the firms had posted losses.
Some stocks traded in the rehabilitation sector, which means the firms are restructuring to improve their financial position, would also be included, it said.
Other stocks include the country's biggest corporate defaulter, Thai Petrochemical Industry, oil retailer Bangchak Petroleum, fixed-line phone firm TelecomAsia and United Communication Industry.
The SEC, which has been investigating suspected manipulation of some stocks, said brokers need not disclose the names of their clients in their reports as the regulator was aiming only to gather information.
Concerns that regulators may impose stricter measures to curb speculation has put downward pressure on the stock market from time-to-time since late last year.
The Thai bourse was the world's best performing stock market last year with a 116.6 percent rise, raising questions about overheating.
On Friday, the main Thai stock index closed down 3.19 percent at 711.15 points due to concerns over the impact from an outbreak of the bird flu virus in Asia. Since the beginning of the year, the index has dropped eight percent.