Low-frequency magnetic field and ion velocity fluctuations in the plasma sheet of Earth's magnetotail are surveyed using data from Active Magnetospheric Particle Trace Explorers/Ion Release Module (AMPTE/IRM) tail passes in 1985 and 1986. The data is divided into 259 ten-minute-long intervals, and the velocity and field fluctuation amplitudes and plasma parameters are determined for each interval. There is no significant dependence of the amplitude of the fluctuations on either the down-tail distance or the distance from the neutral sheet. The amplitudes of both the field and velocity fluctuations are positively correlated with the level of geomagnetic activity as measured by the AE index. The strongest correlation is between the fluctuating velocities and the average ion velocities. These results suggest that geomagnetic activity is the global source of enhanced field and velocity fluctuations observed in the plasma sheet and that velocity shear instabilitiesmay be the proximate cause of these fluctuations.