1. Mallotus oppositifolium is reported to possess medicinal properties and is traditionally used in Cameroon for the treatment of diarrhoea. In the present study, we have evaluated the acute toxicity, in vitro antibacterial and in vivo antidiarrhoeal effects of an aqueous extract of these plant leaves. 2. Shigella dysenteriae A(1) (Sd1)-induced diarrhoeal rats were obtained by oral administration of increasing densities of the Sd1 strain isolated from bloody diarrhoea occurring in East Cameroon. When diarrhoea appeared, rats were treated for 5 consecutive days with 120, 240 or 360 mg/kg extract or norfloxacin (5.7 mg/kg). The weight and frequencies of faeces, as well as the number of Sd1, were assessed during the treatment period and the death rate was recorded. 3. The M. oppositifolium extract was not toxic. In vitro, the minimal inhibitory and minimal bactericidal concentrations of the extracts were 1,172 and 9,375 microg/mL, respectively. In vivo, 12 x 10(8) Sd1 provoked diarrhoea within 24 h, which was characterized by soft or liquid stools, that were moulded, smooth and mucus or blood coated. Diarrhoea went along with an increase in faeces weight and frequency (P < 0.001 by the 3rd day), as well as an increase in the bacterial population to a maximum on the 2nd day after infection (P < 0.05). The death rate was 67% by day 6. 4. Whereas norfloxacin significantly (P < 0.01) reduced Sd1 growth, M. oppositifolium extracts (240 and 360 mg/kg) restored bacterial growth to its initial density and no deaths were recorded. There was a significant (P < 0.05) reduction in stools weight and frequency with 240 mg/kg extract. 5. The results suggest that M. oppositifolium leaves could be a therapeutic alternative for bacterial aetiological diarrhoea in Central Africa, where multidrug supply and access to modern health centres are public health problems.