Bacteriologist Huang Ketai said at least 109 people had died from the plague in Ningbo in November and December 1940.
Dr Huang was giving evidence at a case in which 180 Chinese are demanding compensation and an apology from Japan for the deaths of their relatives during World War Two.
The plaintiffs believe their family members were killed in biological experiments and acts of brutality carried out by Japan's notorious Unit 731.
Dr Huang said the fleas, a kind not native to the region, were infected with "plague with artificially intensified toxicity", which he added only Unit 731 could do.
Houses burned
Lawyers for the plaintiffs say the fleas were dropped by air in a mixture with wheat.
"Obviously, the outbreak was deliberately created," Dr Huang added. "It perfectly matches the area and the timing of the Japanese military's wheat dumping."
The bubonic plague is normally spread by fleas on rats, but only one dead rat was found, he added.
Also unusual was the fact that the outbreak was confined to such a small area.
Dr Huang said infected houses, hospitals and other buildings in Ningbo were burned and had to be left untouched for decades.
'Japan should apologise'
The lawsuit, filed in 1999, claims at least 2,100 people were killed in germ-bombings and other biological experiments by the unit and its affiliates.
After decades of denial, Japan has now admitted that Unit 731 did exist, but it has refused to confirm its activities.
Xuan Wang, a lawyer for the plaintiffs, said they were demanding Japan admit it carried out biological warfare and apologise.
They are also asking for 10m yen ($86,670) each in compensation for the "mental suffering [caused by] biological warfare".
The hearing at the Tokyo District Court is expected to continue for months.
"I have devoted almost my entire life to the flea bombing probe," Dr Huang said in comments quoted by AP news agency.
"Japan's government should at least apologise and compensate for our sufferings."