The occurrence of leaf movements during the daytime is widely known in the leguminous plant canopy. We observed in the soybean field that leaflets moved not independently but that the trifoliolate compound leaf moved as a unit in response to the direction and the intensity of sunlight. Time course changes in trifoliolate leaf movements of soybean, grown in the pot and the field, was investigated by measuring the area of the triangle which connects the three leaflet tips (ΔT), the distance between left and right leaflet tips and terminal leaflet inclination angle. In the early morning, leaflets were already inclined upward, and ΔT reached a minimum value at 10 a.m., followed by a gradual increase in ΔT. As the leaflets became horizontal at dusk, ΔT reached nearly 100%. At the onset of darkness, every leaflet dropped their tips, and ΔT decreased rapidly with the downward (nyctinastic movement). In spite of the darkness, leaflets began to rise in the middle of the night, and turned upward at predawn. The tip of the terminal leaflet, which was oriented north, moved along with the sun movement in a circle from east to west. By measuring the changes in ΔT described above, both heliotropic and nyctinastic movement during the daytime and nighttime could be quantitatively determined. Further, decrease in ΔT within the upper layer of soybean canopy meant that more sunlight penetrated to the lower layer. © 1994, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.