A check comes unexpectedly from your mutual fund company. … What a nice windfall. And just in time for the holidays.

Well, it’s your money, obviously. But before you race out to spend that annual “distribution” check, think about sending it right back where it came from.

These distributions can come anytime, but most fund companies send them out in November and December. They include cash that has built up in the fund from various sources, such as interest earnings and dividends received from stocks.

They also include profits the fund earned from stocks, bonds or other holdings the fund manager sold during the year. Profitable sales are totaled, money-losing sales are subtracted and the net profit is divided by the number of fund shares in circulation.

The result is a per-share payment that, under federal law, must be sent to the fund’s shareholders by year-end.

If you are a retiree, you may intend to count these distributions as annual income to be used for ordinary expenses. In that case, go ahead and deposit the check in your bank account or money market fund.

But if you are an investor who is trying to get as much growth as possible, reinvest the distributions.

In fact, if you invested in the fund because you were impressed by its long-term returns, note that those figures assumed all distributions were reinvested — that is, used to buy more shares of the fund. By spending the money instead of investing it, you’ll reduce your long-term gains.

The easiest way to reinvest distributions is to order the fund company to automatically use them to buy more fund shares.

If you want to invest in another fund run by the same fund family, you could tell the fund company to make that investment. And if you don’t have enough cash to meet the initial-investment minimum required for the second fund, you could let the money build up as cash — in a money market fund, for example.

Or you could have a check sent to you, and use it to invest with a different company. You can even have the money sent directly from one firm to another. They can tell you how.

The important thing is to keep that money working.

Be sure to keep taxes in mind. Distributions received from a fund held in a taxable account are subject to income or capital gains tax, or both, even if you reinvest the money automatically. There’s no immediate tax if the fund is held in a tax-favored account such as a 401(k) or IRA.

A distribution received this fall will be taxable this year, with the amount figured in the return you file next April. The fund company will send you a form in January with details on the distributions.

When stocks were soaring in the late ’90s, many investors received whopping distributions, and thus faced burdensome taxes. Distributions have been much smaller in the past few years because stocks have not done as well.

But there can be exceptions. So before investing in a new fund this fall, call the fund’s toll-free number and ask what distributions will be paid and when.

Unless you think the fund’s share price is going to soar before the distribution date, postpone the purchase until the distribution has been paid. That way you won’t be stuck with a tax on the distribution.

Doesn’t that mean you’d miss out on the payment? Yes, but you wouldn’t lose anything because a fund’s share price drops when the distribution is made, reflecting the fact that cash has been removed from the fund’s accounts.

All else being equal, a fund that pays a $1-per-share distribution experiences a $1-per-share drop in price at the same time. Hence, the distribution doesn’t make investors any richer; it just moves money from one pocket to another, creating a tax bill in the process.

By investing after the distribution, you get the lower share price and avoid the tax.