A disposable bioreactor for perfusion cell culture. Cells are grown in a plastic bag that is rocked and aerated on a mechanical platform. The bioreactor bag contains a filter that allows liquid to be removed from the bioreactor-without losing cells. Nutrients may be added through another port. The perfusion filter is constructed such that it can move freely on the liquid surface. The filter is flicked rapidly across the surface as a result of the rocking motion of the bioreactor and this tangential motion of the filter keeps it from clogging. A weight-based control system regulates feed and harvest rates and allows weeks of continuous operation. This invention has numerous applications in biotechnology and medicine.