The China Securities Regulatory Commission (CSRC) said it is investigating two shareholders of Sichuan Hongda Co Ltd over their sale of stock coming coming out of a holding period, the first such probe since new rules were issued on April 21.

The shareholders are Sichuan Pingyuan Industrial Development Co Ltd and Mianyang Yiduoyuan Real Estate Development Co Ltd.

On April 29, the Shanghai Stock Exchange found that the two companies sold 7.514 million and 6.962 million shares respectively in Sichuan Honda, equivalent to 1.46 percent and 1.35 percent stakes. The transactions were not conducted through block trades, as required by the new rules.

Block trading is mandatory in cases where more than 1 percent of a listed firm's shares emerging from lockup are expected to be sold within a month.

The Shanghai Stock Exchange has frozen the two companies' trading accounts from April 30 to May 28 in order to prevent further trading in Sichuan Hongda shares.