The U.S. trade deficit shrank to $9.89 billion in April, the lowest level since December 1984, the government reported today.

The Commerce Department said that a steep drop in imports helped push the deficit down by 15.5 percent from the revised March figure of $11.7 billion.Many analysts had feared that the deficit would widen in April, so the report showing it actually shrank to its lowest level since late 1984 sent investors scrambling to buy dollars and U.S. stocks and bonds. The buying wave pushed the dollar up sharply against key foreign currencies, wrenched down interest rates in the bond market and gave an early boost to Wall Street, where stock prices jumped.

The key Dow Jones industrial average of 30 stocks rose more than 20 points to 2,133 in the first hour, the highest range since the stock market crash in October.

"This is great news all the way around," said Jay Goldinger, a Los Angeles investment counselor. "We should heave a great sigh of relief. Everyone was prepared for a hurricane and we got bright sunshine and not a cloud in the sky."

At the White House, Reagan was asked about the trade figures during an Oval Office picture-taking session with Italian Prime Minister Ciriaco De Mita.

"It shows that we have continued to bring down the trade deficit," he said. "It has been on a pretty continual decline. And it's been brought about this time by not only an increase in exports but a decrease in imports."

Commerce Undersecretary Robert Ortner said at a news briefing that the figures proved America's trade problems were finally on the way to being resolved.

"These figures are very encouraging," he said. "We are continuing to get an improvement in trade."

The government said that Americans' appetite for imports declined by 6.4 percent to $36.1 billion in April. Exports were also down, but by a smaller 2.5 percent to $26.2 billion. The trade deficit is the difference between imports and exports.

The improvement gave the country its first single-digit trade deficit since a $9.90 billion imbalance in August 1985. It was the lowest imbalance since an $8.03 billion deficit in December 1984.

The March deficit was originally reported as a single-digit figure as well at $9.7 billion. But that was before the department began adjusting to take into account seasonal factors.

After those adjustments, the March deficit grew to $11.70 billion. Beginning with the April report, the department will report only seasonally adjusted figures in the hope that the adjustments will smooth out some of the erratic swings that in past months have sent financial markets into a tailspin.