Biomethanation of horse mixed with cattle dung in the ratios 0:1, 1:4, 2:3, 1:1, 3:2 (wet wt/wt) was carried out in S-litre digesters for 12 weeks at 25+/-1 degrees C. These mixtures produced 2.7, 4.1, 4.7 and 4.4% less biogas, respectively, than the 29.51 biogas kg(-1) obtained from a digester containing pure cattle dung. These mixtures also showed a tendency for solid-liquid separation with increase in horse dung percentage. A one year comparative field study on a 2 m(3) biogas plant run on a 1:4 mixture of horse and cattle dung and a 1 m(3) biogas plant run on pure cattle dung gave average daily gas productions of 21.9 m(3) and 22.6 m(3) biogas kg(-1) wet substrate fed, respectively, showing that 20% replacement of cattle dung cart be made by horse dung for operating family-size biogas plants without much reduction in their gas production or encountering any operational problem.