The principal U.S. victim of the new Nicaraguan trade sanctions appears to be a medium-sized California importing firm that holds an exclusive contract to distribute Nicaraguan bananas in this country.

Joseph F. Dennin, an assistant secretary of Commerce, told a House subcommittee yesterday that the trade embargo will be "a meaningful economic pressure point" on the Sandinista regime, while having virtually no impact on American firms that conduct business with that country.

But that is decidedly not the case for Jack Pandol, the president of Pandol Brothers Inc. in California's San Jaoquin Valley. For the past three years, Pandol's firm has been the sole U.S. importer of Nicaraguan bananas -- the "big ticket" item in Nicaraguan sales here.

At the same time, Pandol is engaged in a joint venture with World Commerce Corp., a Nicaraguan government-owned trading company, to export American banana cartons, plastic bags to cover the bananas, pesticides, fertilizers and other commodities needed for that country's banana crop.

"The only one that gets hurt is me," said Pandol in a telephone interview yesterday. "The Nicaraguans will probably win in this goddamn thing. There's no problem for them to buy these cartons elsewhere. They could get them in Europe."

Pandol, on the other hand, estimates the embargo will cost his company about $1.5 million in brokerage commissions this year, about one-third of his total business. With the sanctions scheduled to take effect this week, he said he and other company employes worked overtime this weekend to unload his last weekly shipment of bananas from a Nicaraguan freighter docked in Los Angeles.

Robert Moore, president of the International Banana Association, said that the overall impact of the Nicaraguan banana cut-off on consumers will be "negligible if anything at all" because that country's bananas account for only about 2 1/2 percent of total U.S. banana imports and there already is an oversupply of the fruit.

"Nobody is going to miss these bananas," said Pandol. "Of course, the ones who are smiling are my competitors like Del Monte and United Brands the distributors of Chiquita bananas . . . . I'm disappointed. But that's my friend, the president who's doing this."

Pandol's complaint is supported by Commerce Department figures showing that banana sales, estimated at $23.5 million last year, were far and away the largest single item in the $58.1 million worth of goods the United States bought from Nicaragua in 1984.

U.S. imports also included about $9.8 million worth of meat, $9.7 million of shellfish, $5.5 million of coffee, and smaller amounts of sugar, tobacco and other commodities, according to department figures.

At the same time, the United States exported $109.8 million worth of commodities to Nicaragua last year, led by pesticides, banana boxes and fertilizers, some of which are handled by Panagua, Pandol's joint venture trading company with the Nicaraguans.

In his testimony yesterday, Dennin said the net impact on U.S. companies will be "quite small" because overall trade with Nicaragua has declined rapidly in recent years as the leftist Sandinista government has aligned itself with Soviet Bloc countries.

A Commerce Department official said yesterday that the department has received about 300 inquiries over the past week from U.S. companies, most of them small and medium-sized firms, asking how the trade sanctions will affect particular sales contracts they have with the Nicaraguans.

But few of those companies will be harmed by the sanctions, the official said, because the administration's regulations issued yesterday allow companies that contracted for deliveries before May 1 to complete them within 30 days and to apply for a special exemption if they cannot do so in that time.

In addition, Exxon, whose international subsidiary has a $15 million oil refinery north of Managua, won't be hurt because the embargos won't affect sales by American subsidiaries in the international market.

One possible exception among big U.S. firms is Monsanto Co., which sells Nicaragua about $7 million to $10 million worth of the pesticide parathyon, produced at a factory in Anniston, Ala., that employs about 300 people.

Stephen Littlejohn, a Monsanto spokesman, said yesterday that "there may be some temporary lay-offs of some employes" at the Anniston plant if the company is unable to find alternative markets for the parathyon.

But, he said, the Nicaraguan cut-off will have no impact on the company overall because the sales from Anniston represent less than 1 percent of Monsanto's total sales of agricultural pesticides and an even smaller portion of overall sales.

"Within the context of our overall business, it's not major," he said.