the present invention relates to improvements of bioreactor bags for cell cultivation . the invention provides an inflatable bioreactor bag for cell cultivation comprised of a top and a bottom sheet of polymer material that are joined along their edges to form a sealed bag , wherein two opposing edges are formed as clamping edges to allow clamping of the bioreactor bag to a rocker type bioreactor , wherein the top sheet is at least 5 % longer than the bottom sheet between the clamping edges . the bioreactor bags provided by the invention avoid formation of undesired wrinkles or creases which otherwise may lead to fatigue of the plastic and eventually fracture .