We propose a mechanism for the cytoplasmic Ca++ oscillator which is thought to power shuttle streaming in strands of the slime-mold Physarum polycephalum. The mechanism uses a phosphorylation-dephosphorylation cycle of myosin light chain kinase. This kinase is bistable if the kinase phosphorylation chain, through adenylate cyclase and cAMP, is activated by calcium. Relaxation oscillations can then occur if calcium is exchanged between the cytoplasm and internal vacuoles known to exist in physarum. As contractile activity in physarum myosin is inhibited by calcium, this model can give calcium oscillations 180 degrees out of phase with actin filament tension as observed. Oscillations of ATP concentration are correctly predicted to be in phase with the tension, provided the actomyosin cycling rate is comparable with ATPase rates for phosphorylation of the myosin light chain and its kinase.