Total assets of money market mutual funds dropped slightly during the week ending Dec. 17, falling $2.96 billion to settle at $3.775 trillion, according to the Investment Company Institute.This marks the first weekly decline in overall assets since late September. During their 11-week run, money fund assets increased by $321.2 billion, or 9.3%.Retail money market mutual fund assets rose by $1.57 billion to $1.284 trillion for the week. Of those, taxable fund assets rose $2.08 billion to $985.75 billion and tax-exempt assets fell $507 million to $297.91 billion.Institutional money market fund assets fell $4.53 billion to $2.491 trillion for the week. Among those funds, taxable assets fell $2.7 billion to $2.306 trillion and tax-exempt assets fell $1.83 billion to $184.7 billion.Money fund assets have increased $630.5 billion year-to-date, or 20%, for what could be their second-best increase ever. The best year was 2007, with a gain of $760 billion in new assets. Institutional assets increased $508.0 billion, or 25.6%, and retail assets have increased $122.5 billion, or 10.5%.The seven-day average yield fell from 0.94% to 0.88%, while the 30-day average yield fell from 1.09% to 1.01%, according to iMoneyNet Inc.'s Money Fund Report.The seven-day compound yield also dropped, falling from 0.95% to 0.89% for the week, and the 30-day compound yield fell from 1.09% to 1.02%.The average maturity of money fund portfolios was 50 days, up from 48.

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