The ultrafiltration (UF) behavior of a surfactant with drag reducing properties was investigated. The UF membrane used was a hydrophilic YM-1 (cut-off 1000) polysaccharide (Amicon). The surfactant used was a non-ionic type N-lauryl-N,N-dimethylaminoxide (LDAO). The total permeation fluxes decreased drastically in the special concentration range just below the CMC. The phenomenon was observed even when the applied pressure was changed. It was concluded that the pre-micelles formed dynamically in this concentration range.