Reversing a trend of outflows, tax-exempt money market funds raked in $3.33 billion to end the week of May 9 with $309.95 billion in total net assets, according to the Money Fund Report, a service of

It marked the single largest inflow activity so far this year, and was the first time the funds gained cash since picking up a modest $137.8 million in the week ended March 21, when total net assets finished at $322.20 billion.

Tax-exempt funds have been plagued by outflows for virtually all of 2011.

The tide began to turn last week when net outflows from tax-exempt money market funds fell to only $302.8 million for the week ended May 2, leaving net assets at $306.62 billion.

That was a sharp contrast to investors yanking $5.17 billion in the week ended April 25 — the largest outflow activity so far this year — when assets fell to $306.93 billion. The enormous outflows came after  investors withdrew $4.70 billion in the week ended April 18, leaving assets at $312.10 billion, as well as $2.25 billion of outflows in the week ended April 11, which dropped assets  to $316.80 billion.

In other activity this week, the average seven-day simple yield for the 480 reporting funds decreased to 0.03% from 0.04%, while the average maturity decreased to 25 days from 26.

Meanwhile, the assets of the 1,138 taxable money funds in the report increased by $19.21 billion to $2.413 trillion in the week ended May 10, following outflows of $2.81 billion that dropped assets to $2.394 trillion in the week ended May 3.

The average seven-day simple yield for the taxable funds remained at 0.02% for a sixth consecutive week, while the average maturity increased to 46 days from 45.

Overall, the combined assets of the 1,618 reporting money market funds garnered $22.54 billion of inflows and finished with $2.723 trillion in total net assets for the May 10 week. That followed losses of $3.11 billion the week before, when total net assets settled at $2.701 trillion.