Embodiments of the invention provide a chamber for a bioreactor, the chamber having a fluid inlet aperture and a fluid outlet aperture disposed at respective different locations of a wall of the chamber, with respect to a normal upright orientation of the chamber the chamber being provided with an upper wall portion defining an upper boundary of the chamber. The upper wall portion has an internal surface having a first portion that is vertically displaced with respect to a second portion. The internal surface of the upper wall portion is arranged to promote expulsion of trapped gas bubbles through the outlet aperture, the first and second portions of the upper wall portion each comprising a sloped portion.