May has been a stellar month for tax-exempt money market funds.

For the fourth week in a row, the amount of money in such funds has surged to a new record high, settling at $520.9 billion for the week ending May 26, according to the Money Fund Report, a service of iMoneyNet.com of Westborough, Mass.

Although the 550 funds experienced a one-week increase of just $1.35 billion, they managed to surpass the prior asset record of $519.6 billion set when they took in $8.11 billion for the week ending May 19. The previous record of $511.9 billion was set after the funds accumulated $11.9 billion for the week ending May 12. That topped previous inflows of $9.98 billion and assets of $499.5 billion for the week ending May 5.

The average seven-day yield dropped 15 basis points to 1.37% from 1.52%, while the average maturity remained unchanged at 22 days.

The 1,292 taxable funds lost $4.32 bilion and settled at $2.95 trillion for the week ending May 27, compared with $5.86 billion of inflows and $2.96 trillion in assets last week.

Overall, 1,842 money market funds lost $2.96 billion, causing assets to decrease slightly to $3.480 trillion after they rose to $3.483 trillion last

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