Two American scientists said here today they have concrete evidence that "yellow rain" in southeast Asia, which the Reagan administration has blamed on Soviet chemical warfare, is excretion by massive swarms of wild honeybees.

Harvard biochemist Matthew Meselson and Yale zoologist and honeybee specialist Thomas D. Seeley told a Harvard University news conference that their recent experience in Thailand proved this.

Seeley said the pair, accompanied by a Thai bee expert, "had the good fortune" to be hit with drops of bee excrement during a five-minute "yellow-rain" shower while they were observing honeybee nests near Lamphun Village in northern Thailand.

"It is a mistake to identify yellow rain as an agent of chemical warfare," said Meselson, who financed the two-week trip earlier this month with part of a five-year, $256,000 award from the MacArthur Foundation. "It is in fact the feces of wild honeybees."

The Reagan administration has charged that "yellow rain"--yellowish spots observed on leaves and rocks--is the residue of Soviet chemical weapons used in Laos, Cambodia and Afghanistan. Refugees have reported that yellow rain causes illness and death.

Some laboratory tests have detected tricothecene mycotoxins, a fungal toxin that could cause internal hemorrhaging, nausea and diarrhea in humans, in yellow-rain samples and in blood, urine and tissue samples from Southeast Asian refugees.

Meselson and Seeley were among a group of scientists who suggested last year that yellow rain was bee excrement, based on high concentrations of bee pollen found in yellow-rain samples, the detection of bee hair in one sample, and the similarity of the spots left by bees in temperate climates. Until their recent trip, however, the scientists were not able to document the phenomenon of mass excretion by honeybees in tropical Asia.

In addition to the yellow-rain shower they experienced in northern Thailand, Meselson and Seeley said, they found 10 other fields of yellow bee excrement. They said they were indistinguishable in size, color, pollen content and shower patterns from samples and descriptions of "yellow rain" by Laotian refugees.

Although it is unclear why the bees defecate in mass flights, the scientists said they believe that the flights occur at the beginning and end of the dry season, the time of greatest flowering in that region, after the bees have ingested large quantities of pollen. That coincides with reported attacks of yellow rain.

When they showed the spots of "yellow rain" to Laotian Hmong refugees at a camp on the Thailand-Laos border, the scientists said "most refugees either did not know what the spots were or erroneously identified them as "chemie" or chemical warfare. Only one refugee correctly identified the spots as bee excrements.

"It is possible that some Hmong, on feeling ill and upon seeing these yellow spots . . . link the two in their minds," Meselson said. He suggested that, when refugees in ill health were asked by westerners whether their sickness was caused by "yellow rain," they would be likely to say yes "once the word is out that yellow spots are associated with illness."

Meselson said he showed leaves spotted with bee excrements to two insect specialists at Harvard, including one bee expert, who also could not identify the spots. The scientists displayed the yellow-spotted leaves at today's news conference.

The two men said they planned to study their "yellow-rain" samples for the presence of mycotoxins, to determine whether such toxins occur naturally in the bee feces. They said that earlier tests for mycotoxins were contradictory and inconclusive.