A new technique, the double strip method, for studying the chemotaxis of myxomycete plasmodia is described. Physarum polycephalum was attracted by the aldohexoses D-glucose, D-galactose and D-mannose and their derivatives 2-deoxy-D-glucose and maltose, thresholds ranging from 0.25 mM (D-glucose) to 5 mM (D-mannose). These sugars competed with each other, a uniform background of one of them inhibiting taxis to the others. Other attractants were N-acetyl-D-glucosamine and mannitol, with thresholds at 1 mM, and fucose (6-deoxy-D-galactose). Although in general only those carbohydrates which could support growth were attractants, there were exceptions such as 2-deoxy-D-glucose; hence metabolism of a compound was not necessary for attraction. In addition, some compounds, such as fructose, could be metabolized but did not attract. At high concentrations (about 100 mM) all the compounds tested, including attractants, could under appropriate conditions cause repulsion, probably through osmotic effects.