Experiments were performed on the Monogonont rotifer Mytilina brevispina var redunca using three Knops media modified to give a high, low, and intermediate calcium balance. Rotifers were also exposed to proportionate levels of the radionuclide 45 calcium in the corresponding Knops media in order that the calcium exchanged throughout the life period could be investigated. Results indicated that rotifers reared on the medium of low calcium balance showed an increase in survival time of 40.2% over the intermediate or control group. Furthermore, this increase was not accompanied by delayed growth or reduced egg production. Exposure of rotifers to 45 calcium in the three media indicated an age related accumulation of calcium, starting at the end of the growth period, whose rate was directly related to the level of calcium in the culture medium, while rotifers that died on a given day of sampling with very few exceptions showed a considerable accumulation of the radionuclide.