Membranous vesicles (microsomes) were isolated from plasmodia of the acellular slime mold, Physarum polycephalum. The microsomes were about 0.2 μm in diameter, and about 10 nm thick. The main protein component of the vesicles had a molecular weight of 100,000 daltons. Calcium ions were taken up by the microsomes only in the presence of Mg2+-ATP. The maximum amount of Ca2+ ions accumulated in the microsomes was 0.24 μmole/mg protein. The Ca2+ uptake was not accelerated by oxalate. The ATPase [EC 3.6.1.3] activity required Ca2+ ions for full activation. The concentra-tion of Ca2+ ions required for half-maximum activation was about 1 μM. The Km and Vm values were 53 μM and 1.6 μmole/(mg·min), respectively. About 0.2 mole of Ca2+ ions was taken up by the microsomes, coupled with the hydrolysis of 1 mole of ATP. The ATPase activity and Ca2+ uptake of the microsomes were not inhibited by sodium azide. Furthermore, electron microscopic examination showed that mitochondnal contamina-tion was slight. These results suggest that a vesicular calcium transport system, analogous to the sarco-plasmic reticulum in skeletal muscle, is involved in regulation of the Ca2+ concentration in plasmodia of Physarum.