Slow sand filtration is wide spread as a rural drinking water treatment process in most of the developing countries, and is well known to become effective after a long period of filtration. This study evaluates the efficiency of granular pozzolans from Djoungo and Koutaba (Cameroon), at the beginning of a slow filtration process, using bentonite as a model colloid, under different physicochemical conditions. Experiments were conducted at various pH (5-9), different ionic strengths (10(-4)-3 center dot 10(-2)M KCl) and various flow velocities up to 0.24mm/s, with a filter grain size of 400-500 mu m. The results show that the pH variation (5-9) has little influence on the retention of bentonite clay when the ionic strength is less than 10(-2) M KCl; whereas conditioning the grains at 10(-2) M KCl and pH 5 improves the efficiency of retention, increasing flow velocity results in a small decrease in retention efficiency. The three pozzolans tested gave similar retention efficiencies.