An optical method was designed for monitoring contractile activities in the plasmodia of Physarum polycephalum when the plasmodia were migrating and just before the formation of sporangia and spherules. A plas-modium at each stage was mounted under an optical microscope so that it could be viewed as a whole organism. The light at the ocular was partially obstructed by the non-transparent plasmodium, reflecting its deformation. The application of attractants (glucose, maltose, phenylalanine) and repellents (KC1, CaC12, LaC13) caused relaxation and contraction of the normal plasmo-dium. Until just before sporulation and sclerotization the contraction rhythm persisted synchronously, whereas the amplitude of the oscillation gradually diminished. The oscillation period remained at a constant level during fruiting, but was prolonged during sclerotization.