Tax-exempt money market funds posted their first decline since mid-December as outflows of $4.35 billion trimmed total net assets to $329.91 billion for the week ended Jan. 17, according to the Money Fund Report, a service of iMoneyNet.com.

The drain more than offsets the previous week’s $2.098 billion gain, which boosted assets to $334.26 billion and marked the funds’ fourth consecutive week of inflows.

Outflows are typical at this time of the new year, according to iMoneyNet.

Total net assets of tax-exempt money funds were nearly the same a year ago, when they settled with $393.62 billion after losing $7.67 billion the week ended Jan. 18, 2010.

The iMoneyNet average seven-day simple yield for the 481 tax-exempt money funds in the report was unchanged this week at 0.03%.

The average maturity increased one day to 31 days. A year ago, the yield held at a record low 0.02%.

The 1,131 taxable funds in the Money Fund Report lost $45.48 billion for the week ending Jan. 18, dropping net assets to $2.394 trillion.

That outflow dwarfed the previous week’s $3.11 billion loss, which dropped taxable funds to $2.440 trillion in assets.

Those losses compare with a year-ago seasonal outflow of $38.19 billion for taxable funds for the week ended Jan. 19, 2010, when they settled with $2.811 trillion.

The iMoneyNet average seven-day simple yield for the taxable funds remained at 0.03%.

The average maturity also was unchanged at 46 days. The year-ago yield was also 0.03% for the week ended Jan. 19, 2010.

Overall, the combined assets of the 1,612 money funds reporting sank by $49.83 billion during the week ended Jan. 18, ­after dropping $1.017 billion the previous week.

Total net assets settled at $2.725 trillion this week, compared to $2.774 trillion last week.

Funds experienced similar outflows during the same period last year, ­losing $45.85 billion in the week ended Jan. 19 and finishing with total net assets of $3.205 trillion.