More than 2,100 cases of a penicillin-resistant strain of gonorrhea have been reported in the first six months of this year, nearly twice the number reported in the same period last year, according to the federal Centers for Disease Control in Atlanta.

The CDC said the total almost surely will be more than 4,000 by year's end. In a report in today's New England Journal of Medicine, the CDC called the situation alarming.

The penicillin-resistant strain, common in the Far East and Africa, was brought to the United States from the Philippines in 1976 by American servicemen. Cases are found in many segments of the population, but are especially common in two groups: the urban poor and prostitutes.

At first there were no more than 400 cases a year, and only 1,099 cases were reported as late as 1980. The number reached 2,734 last year and is growing.

Some areas have had only a few cases, but other areas have had many. New York City counted 663 cases through June, nearly a third of the nation's total. There were 372 cases in Florida, and 15 in the District of Columbia.

"Bitter experience with other resistant organisms" suggests that this form of gonorrhea "may ultimately prevail," wrote Dr. William McCormack, chief of infectious diseases at the State University of New York's Downstate Medical Center in Brooklyn, in the medical journal.

Dr. Eugene Washington of CDC agreed "that's our concern." The resistant strain causes more than half of all cases of gonorrhea in some parts of Asia.

Most of the penicillin-resistant cases are being treated successfully with the antibiotic spectinomycin. But spectinomycin fails in some cases, especially when there is throat infection or another infection, such as syphilis.

Spectinomycin's main drawback is its price, Washington said. It costs public health departments $5 to $6 a dose, compared with 80 cents to $1.20 for various forms of penicillin.

At a time when health departments are strapped for funds, "this is an important consideration," he said.

Some cities are trying a new technique in fighting gonorrhea outbreaks, called a spectinomycin "blitz."

"A blitz means that instead of using penicillin we immediately use spectinomycin in all cases seen," Washington said.