Investors pulled cash out of Bill Gross's Pimco Total Return Fund in August for a 16th straight month.

The flagship fund at Pacific Investment Management Co. posted a $3.9 billion net outflow last month, higher than the $830 million outflow in July, according to fund tracker Morningstar.

August's outflow represents 1.8% of its assets at the end of July, according to Morningstar.

Assets in the Pimco fund fell to $221.6 billion at the end of August, compared with $223.1 billion at the end of July.

The Pimco fund's size has dropped every month after hitting a record high of $292.9 billion in April 2013. The fund remains the world's biggest bond fund by assets.

Asset numbers also take into account performance of the fund, while flow numbers don't.

Mr. Gross is co-founder and chief investment officer at Pimco, which manages $1.97 trillion in global assets as part of Germany's Allianz SE.

Investors have kept a close eye on Pimco since Mohamed El-Erian, groomed by Mr. Gross as a possible successor, quit as chief executive and co-chief investment officer earlier this year. The Wall Street Journal reported in February that Messrs. Gross and El-Erian had clashed openly.

Write to Min Zeng at min.zeng@wsj.com

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