Physarum polycephalum, a slime mold typically found in decaying plant vegetation, is a member of the class/superclass Myxogastridae. P. polycephalum moves through cytoplasmic streaming due to the interaction between actin and myosin within the walls of its plasmodial tubes. Although early studies indicated that calcium has an excitatory effect on growth, more recent studies showed that the effect of calcium on growth is inhibitory. To determine the direction of the effect of calcium on the growth of P. polycephalum, the organism was grown on agar strips containing various concentrations of CaCl₂ for a 72-hour period. Growth was monitored every 24 hours. During the first 48 hours of growth, calcium significantly increased the growth of P. polycephalum. However, in the next 24-hour interval, calcium significantly decreased the growth of the organism. In addition, as the CaCl₂ concentration increased, both the enhancement and inhibition of growth became greater. This suggests that the initial effect of calcium is enhancement and the later effect is inhibition.