THE UNITED STATES is considerably behind its allies in seizing opportunities for reciprocal economic agreements with developing nations.

The European Economic Community's Lome Convention, for example, has forged new aid-and-trade ties with 54 Third World countries. In the "new China" market, the United States' one mining deal pales besides the headstarts of Germany and Japan in building everything from coal mines to steel mills.

But the U.S. lag is most striking when compared with other industrial nations' bilateral swaps of exports for oil and gas.

Japan is building Iraq's natural and liquefied gas industries in exchange for oil, while Peru is already paying for its Japanese-built pipelines at a price of 250 million barrels of oil. France has announced a policy of tying its oil purchases to exports of French products, and Germany and Japan are negotiating long-term deals to export technological assistance and capital goods for oil.

For the United States, Mexico should be an obvious choice for such agreements, but we recently rejected Mexico's bid to sell us natural gas at a lower, price than we pay for Algerian and Indonesian gas. Massachusetts Sen. Edward M. Kennedy echoed the dismay of others in Congress when he noted how "President Lopez Portillo turned instead to Japan for technology, financing and oil sales."

Our refusal prompted Mexico to convert the gas to domestic uses, thereby freeing oil for export to Japan. The Japanese loaned Mexico $1 billion to build pipelines and port facilities so that Mexico can ship the oil to Japan in repayment of the loan.

Similarly, France and Mexico have signed a deal providing France with 100,000 barrels of Mexican oil a day, paid for by French exports.

The advantages of the United States arranging a similar deal with Mexico are obvious. It would give us an additional, stable supply of oil without worsening our already deep trade deficit, stimulate our economy, create jobs and reduce unemployment and related government payments.

The United States, for example, could offer to buy a scheduled amount of Mexican oil leading up to 2 million barrels a day by 1985. Mexico in return would receive a line of credit with which to shop in this country for the best buys on capital goods and technology. Indeed, Mexico and the United States are geographically so close that lower transportation costs -- compared with shipments bound for Europe or Japan -- would mean that the Mexicans in effect could gain up to an extra $1 profit for each barrel they send us.

We could sweeten this package with special financing arrangements, possibly including an adjustment in Mexico's debt repayment schedule. Even more important would be a move toward serious negotiations to resolve such difficult questions as immigration policy and water rights to the Colorado River, the key issues in U.S.-Mexican relations.

If such a swap deal could be negotiated, it would mean new U.S. exports to Mexico of almost $11 billion a year, creating an estimated 400,000 jobs in this country. Since every jobless worker costs the federal government about $5,000 in unemployment compensation and other payments, taxpayers could conceivably save more than $2 billion in such aid. That would translate into a $2.74, or 20 percent, premium on each barrel of Mexican oil.