SANTA ANA, Calif. — A federal jury here is being schooled in the nuances and ambiguities of the Chinese language as defense lawyers challenge translations that the government says put the defendant, Chi Mak, at the center of a long-running Chinese espionage operation in the American defense industry.

In testimony yesterday, a linguist for the defense disputed the FBI’s interpretation of wiretaps and surveillance recordings that the prosecution claims are incriminating evidence that Mr. Mak was a covert agent.

Prosecutors contend that a phone recording captured Mr. Mak’s brother calling his handler in mainland China and using a code phrase to identify himself as affiliated with a spy ring called “red flower of North America.”

However, the defense linguist, Eddie Yip, said the words uttered in the recorded call are not “hong hua,” which could mean red flower, but “hong fa,” which could mean ” Wang prosperity,” the name of a Mak family company.

Asked how sure he was that the Mandarin word for flower was not on the recording, Mr. Yip said, “100%.”

Mr. Yip also undercut the government’s interpretation of a noisy surveillance recording made in Mr. Mak’s car five days before his brother and sister-in-law were arrested at the Los Angeles International Airport as they prepared to carry to China a disk containing encrypted data on submarine propulsion technology. The prosecution contends that Mr. Mak can be heard discussing a CD and later using Chinese words meaning “encrypt.” However, Mr. Yip said the phrase he heard has a far more innocuous meaning, “all of them.”

A defense lawyer, Marilyn Bednarski, also played the recordings aloud to demonstrate to jurors that the sounds are often murky and hard to make out. Classical music can be heard in the background of the recording from Mr. Mak’s car.

The defendant chuckled, and some jurors looked at him sympathetically as one of his loud yawns was played again and again over speakers in the courtroom.

The defense also stressed that sounds in Chinese often correspond to multiple written words or meanings, making context essential to accurate interpretation.

FBI linguists scoffed in the gallery as Mr. Yip took issue with their interpretations of the key phrases. A prosecutor, Craig Missakian, brandished Chinese dictionaries and suggested that when translating a phrase found in another recording, Mr. Yip overlooked translations that mean “secret” or “confidential” and seized on others that mean “compressed” and “made tight.”

“I cannot type the whole dictionary into one page,” Mr. Yip replied tartly, before noting that he had listed encryption as one possible translation.

Mr. Missakian also challenged the defense linguist’s credentials. While Mr. Yip, a former corruption investigator in Hong Kong, claimed to be fluent in Mandarin, Cantonese, and Shanghainese dialects, he conceded that he did not pass an oral language examination administered for the California courts. He said continues to work in the courts because the test is not yet mandatory.

Mr. Mak, a naturalized American citizen who worked as an electrical engineer for defense contractors in southern California, is charged with violating export controls, acting as an unregistered agent for the Chinese government, and lying to investigators. He has not been charged formally with espionage. Four other members of his family have been charged in the case and are expected to go on trial separately next month.

Jurors also heard yesterday from an engineer who worked with Mr. Mak in the 1980s, Suresh Gupta. He said he worked regularly on a submarine project for the Navy, known as the USS Dolphin, even though he was not an American citizen.

Mr. Gupta said could he bring foreign nationals onto the submarine without any scrutiny and took no particular precautions with the plans. “I was supposed to design the whole thing from home,” he said. Prosecutors noted that the USS Dolphin is not a nuclear attack submarine like the ones Mr. Mak helped design.