1. The rates of influx and efflux of [32P]orthophosphate (Pi) have bben measured in single muscle fibres of the spider crab, Maia squinado. 2. Rates of influx of 32Pi from salines containing 0 . 11 to 0 . 37 mM-Pi ranged from 0 . 11 to 0 . 41 p-mole cm-2 sec-1. 3. After injection of [32P]orthophosphate there was an early rapid phase of 32P efflux which was maximal after about 5-10 min, then a gradual decline until a low steady-state efflux rate was established after about 100 min. The rate of decline of 32P efflux was similar to the rate of equilibration of injected 32Pi with arginine phosphate and ATP. 4. The steady-state rate of 32P efflux was typically about 0 . 70 p-mole cm-2 sec-1 when the sarcoplasmic Pi concentraion was 5 m-mole kg-1. Exposure of mucle fibres to conditions which caused contraction and increased the sarcoplasmic concentration of Pi also increased the rate of 32P efflux, which appeared to be linearly related to sarcoplasmic Pi concentration. 5. The results are compared with previous measurements of phosphate fluxes in nerve.