The properties of humic acid fouling in cellulose acetate microfiltration membranes and the response of the zeta potential, zeta(obs), of the membrane to the fouling were studied with two dead-end filtration modules connected in series. zeta(obs) was monitored during the filtration by streaming potential measurement by pulse pressure mode. The fouling was caused by particulate humic acid at membrane surface and dissolved humic acid did not cause fouling. The fouling observed in the membrane having pore size of 0.22 mu m was the cake filtration and those in the membranes having pore size of 0.45 and 0.8 mu m were the pore blocking filtration followed by the cake filtration. The zeta(obs) changed during the filtration responding to the increase in the filtration resistance. The responses of zeta(obs) to the fouling can be elucidated in terms of the location of pressure drop and the local zeta potential.