The effect of surfactant solutions on the flow past a circular cylinder was investigated in the Reynolds number (Re) ranging between 10 and 105 by measuring the drag and by flow visualization. Surfactant solutions were aqueous solutions of oleyl-bihydroxyethylmethyl-ammonium-chloride in the concentration range of 50 to 200 ppm, to which sodium salicylate was added as a counterion. It was clarified that the drag coefficient of the cylinder in surfactant solutions increases, comparing that in water in a lower Re range for constant cylinder diameter. On the contrary, the drag coefficient decreases in higher Re range. The Re range in which drag reduction occurs increases with increasing diameter. The flow visualization results show that the drag increases because of the existence of the wide stagnation zone around the cylinder. This zone disappears in the Re range in which drag reduction occurs. In this case, the separation point moves rearward and the velocity defect in the wake becomes smaller. (c) 2005 American Institute of Chemical Engineers AIChE J, 52: 49-57, 2006.