This study was conducted to evaluate the anti-diarrhoeal activity of Bacillus clausii strain UBBC 07 in patients suffering from acute diarrhoea. A total of 27 patients (average age of 35.44±8.08 years) with acute diarrhoea were included in a prospective, Phase II clinical study after informed consent and ethical committee approval. The criteria included for all subjects were ≥3 loose stool motions within 24 hours and for more than 7 days. All patients were assigned to receive one capsule of B. clausii strain UBBC-07 (containing 2×109 cfu) two times a day for a period of 10 days. Efficacy assessment of duration of diarrhoea, frequency of defecation, abdominal pain and stool consistency were tested on days 1, 3, 6 and 10. Safety was evaluated by assessing the incidence and type of adverse effects such as increase in blood pressure and pulse rate, physical examination and clinical laboratory tests, i.e. complete blood count, serum glutamic pyruvic transaminase, serum creatinine, and stool examination and microscopy, on day 1 and day 10. The results of this study clearly showed that the mean duration of diarrhoea decreased from 34.81±4.69 to 9.26±3.05 (P<0.0001) min per day, the frequency of defecation also decreased from 6.96±1.05 to 1.78±0.50 (P<0.0001) times per day, abdominal pain decreased from 3.22±0.93 (severe) to 0.74±0.71 (absent) (P<0.0001), and stool consistency improved from 3.93±0.38 (watery) to 1.22±0.42 (soft) (P<0.0001). No significant change in safety parameters were observed during treatment. This study shows that the B. clausii strain UBBC-07 can potentially be effective in alleviating the symptoms of diarrhoea without causing any adverse effects.