Fund manager Bill Gross, who in April said a bet against German bunds is the "short of a lifetime", said the next new opportunity is wagering against Chinese stocks.

Gross's bet against bunds is now "happening", he wrote in a Twitter message on Wednesday. "Up next: the China Shenzhen index. Not just yet ..."

The Shenzhen Component Index has surged more than 100 percent this year as the benchmark stock market experiences the world's biggest rally amid a flood of initial public offerings and record amounts of new investors.

The Shenzhen index's price-to-earnings ratio is 74.5 times, compared with 24.6 times for the Shanghai Composite. The Standard & Poor's 500 Stock Index P/E ratio is 18.7 times.

"Future growth doesn't justify these levels and technically it has similarities to a chain letter," Gross wrote in an e-mailed message.

"Chinese retail investors are opening up new accounts at a record pace and fueling the almost hyperbolic rise in prices," he wrote. "At the same time, Chinese exports are slowing and its currency - the yuan - is becoming increasingly uncompetitive to other Asian currencies because it is tied to the dollar."

Gross, who advised selling short the 10-year German bund on April 21, failed to profit even as the move he had forecast came to be. He later acknowledged that his forecast was "well-timed but not necessarily well-executed".

Gross, who runs the $1.52 billion Janus Global Unconstrained Bond Fund, made his prediction on the German bunds when it yielded about 10 basis points. Over the next month, yields climbed to 64 bps as prices fell. His fund lost 2.5 percent in that period as he wagered the bund would trade in a narrow range instead of betting all-out against the debt, data posted on Janus's website show.

Yields on Germany's 10-year sovereign debt reached the highest level of this year on Thursday at 88 bps, as European Central Bank President Mario Draghi told investors to get used to volatility. Those declines left investors in European bonds sitting on a year-to-date loss for the first time in 2015.

Gross was chief investment officer at Pacific Investment Management Co until September, when he jumped to Janus.