Diarrhoeal management practices are unsatisfactory in India especially in the slum areas. Dearth of information regarding physicians' diarrhoea-related knowledge and practice in India ncessitated this cross-sectional study of allopathic practitioners in the slums of Kolkata, to assess the distribution and interrelationship between physicians' characteristics, knowledge and practice regarding diarrhoea. A total of 264 randomly selected consenting practitioners were interviewed using a field-tested questionnaire. Nineteen percent had good overall knowledge, 49% and 80% prescribed antibiotics to diarrhoea and cholera patients, respectively, and 55% advised stool examination for every case. Qualified and Government physicians had better knowledge regarding diarrhoea [MBBS: odds ratio (OR) 5·96, P < 0·001; postgraduates: OR 9·33, P < 0·001; Government physicians: OR 11·49, P < 0·0001] and were less likely to prescribe antibiotics for all diarrhoea cases (MBBS: OR 0·30, P = 0·002; postgraduates: OR 0·20, P < 0·001; Government physicians OR 0·24, P < 0·029). Better knowledge was associated with a lower likelihood of prescribing antibiotics for diarrhoea (OR 0·72, P < 0·001), cholera (OR 0·78, P = 0·027) and investigative procedure (OR 0·85, P = 0·028). In the slums of Kolkata, diarrhoea-related knowledge and practice were poor with the exception of qualified physicians, hence an improvement in the knowledge of pharmacists and unqualified practitioners is necessary for the overall improvement of diarrhoeal management in these slums.