Mario Castillo is bewildered.

Castillo and other members of the Lee Gardens tenants association felt as if their efforts had paid off last week when the Artery Organization of Bethesda agreed to sell 364 units in the north section of the complex to a nonprofit Arlington housing group.

The deal would set aside 200 units for low-income tenants. The sale is not final until Artery reaches an agreement with its lenders.

But Castillo and other tenants from the south side of the complex, who were considered the most active in championing the sale to the Arlington Housing Corp., now find they may not get the coveted subsidized units.

Because of regulations governing federal housing subsidies, tenants from the north side of the 961-unit complex will have first priority for the low-income apartments, all of which are located there.

"I'm very disappointed, disheartened," Castillo, secretary of the tenants association, said through an interpreter. "We had started this fight to get something better for all the community and for ourselves. After all this, we're the ones put aside, left out."

Federal regulations vest the tenants in the north section, which will become a separate housing project, with special rights, including first preference for subsidized units, said Arlington County Manager Anton S. Gardner.

Gardner said the county had asked the U.S. Department of Housing and Urban Development for permission to give residents in the south part of Lee Gardens second priority for the units.

HUD also has been asked to allow priority to be given to south Lee Gardens tenants for housing vouchers that will subsidize their rents at privately owned apartments elsewhere in the county, he said.

"I understand how it feels . . . and how personally wrenching it is to some individuals," said Gardner.

This has not mollified tenants living in the south side of the complex, some of whom were turned down when they asked permission to move to the north side of the complex to receive priority for the subsidized units. A person familiar with the negotiations said that such a move would have unfairly favored some tenants of south Lee Gardens over others.

"Now we have to move," said Patricia Rodriguez, vice president of the tenants association and a resident of the south part of Lee Gardens. "We worked very hard to help them. We thought they would help us," she said of all the parties involved in the transaction. "I feel very bad."

The Arlington Housing Corp. has begun surveying families in north Lee Gardens to determine which are eligible for federal subsidies and the sizes of the units they will need.

It appears that 19 families are too large to be accommodated at the complex under state occupancy rules, said Wayne E. Rhodes, development coordinator for the county's housing division. Those families will have to move and will be offered relocation benefits, said Rhodes.

A similar, painful sorting out process is taking place at Alexandria's Dominion Gardens. That complex is also owned by Artery, which last week agreed to set aside for five years 104 of 416 units for low-income tenants who qualify for federal housing subsidies.

The agreement was approved by a federal judge as part of a settlement of a lawsuit alleging housing discrimination.

Some Dominion Gardens tenants complained at a court hearing last week that their settlement results in too few units being set aside for them and that occupancy restrictions will force large families from the complex. Also, some tenants complained that some small families can remain at the complex until they find suitable replacement housing, yet large families have been given a deadline to move.

"I was so disappointed in the settlement," said the Rev. Eldridge Harmon, a tenant at Dominion Gardens since 1985.

Attorneys for the tenants said the majority of residents were interested in settling the case.

U.S. District Judge Harold H. Greene, who approved the settlement, said in his ruling that state occupancy laws limiting the number of tenants in a unit would have been enforced even if the tenants had been victorious in their suit.

To qualify for a subsidized unit, a tenant must receive from the city a federal Section 8 voucher that limits a tenant's rent to 30 percent of his income, said Barry Goldstein, an attorney for the NAACP Legal Defense and Education Fund who represented the tenants in the court case.

Only families of four or fewer will be eligible for the subsidized units because of the occupancy requirements, he said. So far, 129 familes have applied for the vouchers, he said. Some may not qualify financially, but if there are more applicants than units a lottery may be held.

There also may be room at the nearby Bruce Street apartments, where 68 out of 275 units are being set aside for low-income tenants as a result of a separate settlement with a different developer, said Goldstein.

"This is a very tough situation," said Goldstein.

"What we got was 104 units and a substantial increase in relocation benefits" for some Dominion Gardens families, he said. "We didn't solve all the housing problems in Northern Virginia."