WASHINGTON (AP) _ The Reagan administration and the plastics industry are opposing attempts to outlaw plastic six-pack holders in favor of yokes made of biodegradable materials, a change that environmentalists say would protect wildlife.

Administration and industry officials told a House Merchant Marine and Fisheries subcommittee Tuesday that more research is necessary to determine what impact the by-products of a degradable holder would have on marine life. But the environmentalists say legislation is needed quickly to save seabirds and marine mammals that are killed by the discarded plastic rings.

Albert M. Manville II, a biologist for Defenders of Wildlife and chairman of the Entanglement Network coalition, said plastic six-pack containers can remain in the environment for ''hundreds, perhaps thousands of years,'' killing wildlife that become entangled and choke to death, or mistake the yokes for food and strangle or die from starvation.

''The six-pack ring epitomizes the plastic pollution problem - a highly successful product that is functional and durable, and yet ugly and deadly when improperly discarded,'' said Rep. Gerry E. Studds, D-Mass., chairman of the subcommittee on fisheries and wildlife conservation.

But Sylvia Lowrance, director of the Environmental Protection Agency's solid waste division, warned that the proposed law has ''the potential for replacing one problem with another.'' And she noted that even degradable yokes can stay in the environment for several months before breaking down.

Robert Smith, deputy assistant director at the Fish and Wildlife Service, said his agency opposes the legislation because ''we do not have information that indicates six-pack yokes are a major threat to fish and wildlife resources.''

''They probably are not a major threat to wildlife population,'' Smith said.

Environmentalists disagreed with that assertion and urged immediate action.

''Enough is enough,'' said Manville. ''When is there sufficient information?

''The majority of what is seen and reported involves animals on shore. These reports probably represent only a fraction of what is entangling fish and wildlife on the open oceans.''

The United States produced 10 million tons of plastic waste in 1986, about 7 percent of the 158 million ton total, according to Lowrance. Projections show the plastic trash total rising to 16 million tons by the turn of the century.

The bill by Sen. John H. Chafee, R-R.I., was passed by the Senate in May and is pending before Studds' subcommittee and the House Energy and Commerce subcommittee on transportation and tourism.

Rep. Thomas A. Luken, D-Ohio, chairman of the commerce subcommittee, said ''there may be reason to question the need for congressional action'' in light of states' responses.

Sixteen states have enacted similar laws, beginning with Vermont in 1976.