A new phase in the struggle for black majority rule in Africa begins Tuesday when leaders of nine southern African states meet here to chart future economic dealings with South Africa.

The nine nations, an expansion of the five front-line states that helped press the war against Rhodesia, are expected to agree to a declaration calling for regional cooperation to lessen dependence on South Africa.

The unspoken underlying theme, however, is likely to be that the countries will have to accept economic coexistence with South Africa until they can improve their cooperation in such fields as transportation and communications.

The participation of Rhodesia, as the new nation of Zimbabwe, will mark the end of the struggle for black independence there and put South Africa at the forefront of the group's concerns. There is widespread recognition that, unlike in Rhodesia, black rule in South Africa will have to come about through internal agitation and resistance rather than through an externally supported guerrilla war against such a powerful foe.

Southern Africa's stake in avoiding such hostilities, which could lead to closing of borders and trade routes, is clear: seven of the nine nations meeting here have more trade with South Africa than with all the rest of Africa combined.

South African currency freely circulates in two of the groups's new member states, Swaziland and Lesotho.

The economy of the other new member, Malawi, is also closely tied to Pretoria, thus placing the new lineup facing South Africa within a more moderate spectrum.

Zambian President Kenneth Kaunda called the summit earlier this month after the election of Robert Mugabe as prime minister of an independent Zimbabwe. His vicotry assured a settlement of the 15-year-old Rhodesia conflict acceptable to the original front-line states of Mozambique, Zambia, Botswana, Angola and Tanzania.

After years of concentrating on Rhodesia, it will be the first time the leaders have held a summit devoted to economics.

The expansion of the front line, if only for economic purposes, is a blow to South African Prime Minister Pieter Botha's hopes to form a rival "constellation of states" in southern Africa.

Regional economic cooperation could help individual nations to withstand South African pressure to join its grouping. Lesotho, Swaziland and Botswana, which are already in a customs union with South Africa, are particularly vulnerable.

A well-informed Zambian official said recently, "South Africa is the most competent country for us to deal with economically, but the most objectionable politically."

Zambia, long a host for anti-South African groups, seems to be handling the problem in a way that will probably be a pattern for most of the nations represented at the summit. It condemns South Africa politically while continuing economic relations.

This month Zambia became the first independent African nation to open a direct air link with South Africa, even though Pretoria's troops continue to cross into Zambia to battle guerrillas fighting for the independence of South African-controlled Namibia.

In 1978, Zambia's trade with South Africa was triple that with all the rest of Africa. That difference is probably much larger now because of imports of more than 100,000 tons of badly needed grain. One Zambian said the country would have had a hard time surviving the food crisis without the aid of our "comrades in South Africa."

Another Zambian official said, "the realities of the region must be taken into account. The front-line states are unable to do without South Africa." i

If the nine nations at the summit "don't come out for economic coexistence, we will be pushed into a position of confrontation," he said and added that this could lead to some defections to the South African constellation.

Another indicator that the summit will acknowledge the need for coexistence, although probably couching it in less direct language, came this weekend from Zimbabwe's new finance minister, Enos Nkala.

"No one can ask us to hang ourselves to help our friends," he said. "If we commit suicide how do we aid our allies" in South Africa seeking black rule? No state at the meeting will "cajole us into instituting economic sanctions against South Africa," said Nkala. Just last month he was barred from campaigning in the Rhodesian elections because of alleged warlike statements.

Despite South Africa's economic dominance, Rhodesia's independence next month should provide the opportunity for increased regional cooperation beyond Pretoria's influence. Reopening of road and rail routes long closed because of the seven-year guerrilla war will allow most of the countries to use Mozambique's ports as an alternative to those in South Africa.

During years of economic sanctions, Rhodesia developed its own manufacturing capacity, allowing it to compete with South Africa for markets among Salisbury's new allies.

For heavy industrial equipment, cars, trucks and high technology items, however, South Africa is still bound to be the most economical source for the front-line states. One problem for them is that many of their own exports, such as agricultural products, compete with each other.