1. The total Mg in isolated fibres of Balanus aquila was 10-5 m-mole/kg wet wt. 2. The intracellular free Mg was measured by a null point method using Eriochrome Blue as an indicator of free Mg, and internal dialysis with solutions of varying ionized Mg concentrations. The results indicated a free Mg of 6 mM or 4-2 m-mole/kg wet wt. in the intracellular water immediately surrounding the dialysis capillary. 3. The ATP concentration was estimated to be 4-9 m-mole/kg wet wt. 4. A tentative partitioning of Mg among various intracellular constitutents based on present data combined with published work by others is (m-mole/kg wet wt): free, 4-2; MgATP, 4-2; myofibrillar bound, 1; residual (presumably bound to arginine phosphate and phosphate) ca. 1.