"Strain of Vibrio cholerae Ogawa was found in the stool samples of four patients who were among many other people admitted with symptoms of diarrhoea," said M.M. Deka, chief superintendent of the Gauhati Medical College Hospital.
Authorities, however, said the strain was not "very virulent" and there was no need for panic.
"At least 25 people were admitted to various hospitals with symptoms of diarrhoea and we are taking no chances. Thorough pathological tests are being carried out on the patients," a health department official said.
The district administration has sounded a general alert and launched a massive cleanliness drive, including distribution of water purifying tablets in the affected areas.
"All precautionary measures are being taken and the patients are being provided with the best of medical care," said Avinash Joshi, Kamrup district magistrate.
A 24-hour health control has been set up to monitor the situation.
"We appeal to the city residents to immediately come to the nearest hospital if someone is found to be suffering from diarrhoea," the health official said.
In its most severe forms, cholera is one of the most rapidly fatal illnesses known - a healthy person may die within two to three hours of the onset of symptoms.
The symptoms include those of general stomach upset and massive watery diarrhoea, including terrible muscle and stomach cramps, vomiting and fever.
Cholera is transmitted through ingestion of faeces contaminated water loaded with the cholera bacterium. The source of the contamination is typically other cholera patients when their untreated diarrhoea discharge is allowed to get into waterways or into groundwater or drinking water supply.
Any infected water and any foods washed in the water, and fish living in the affected waterway can cause infection.