NEW YORK ( TheStreet) -- Citigroup ( C) is adding annual fees to many popular credit cards a week before the CARD Act goes into effect.

Some Citigroup cardholders are receiving letters about a $60 annual fee that is being attached to their account effective April 1. If consumers make $2,400 in purchases during the year, the annual fee will be credited to their account. Only about 20% of credit cards in the U.S. have annual fees, according to data compiled by LowCards.com.

It appears that Citigroup's test of adding an annual fee to a small share of customers in August 2009 proved to be successful for the issuer, which competes with Visa ( V), MasterCard ( MC) and Capital One ( COF).

At that time, Citigroup began charging some cardholders an annual fee of $30 to $90 unless they spent at least $2,400 a year. Now a far greater number of customers are receiving this notice.

"The reason we are making this change is to maintain the quality of our service amid the rising cost of doing business," said Ken Stork of Citibank in a letter to the cardholders receiving this notice.

Citigroup's decision may be a sign of things to come. Issuers such as Citigroup have been squeezed weaker consumer spending, higher default rates and the enactment of the CARD Act, which puts limits on some interest-rate increases. Adding annual fees is a way to easily generate revenue.

For consumers who spend at least an average of $200 a month on their Citigroup cards, the fee may not be a big deal. Those who own credit cards for emergencies will now have to pay for that safety.

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