First Union Corp.'s stock fell in heavy trading Wednesday after Banco Santander, a Spanish bank, announced that it sold over 3.5 million shares.

Banco Santander reduced its ownership in First Union to less than 10% from 11.3%, to improve its "regulatory capital treatment," said First Union.

More than six million shares traded during the day, about 11 times the average daily volume, sending shares of Charlotte, N.C.-based First Union down $1.375 to $64.

Analyst Thomas K. Brown of Donaldson Lufkin & Jenrette said the sale may have been prompted by First Union's announcement earlier in the week that its credit card chargeoffs would hold steady through the rest of the year.

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