The present invention is a disposable bioreactor formed of molded plastic. The bioreactor is presterilized and has a top and body sealed to each other. One or more ports are formed in the top and side of the housing. Preferably at least one port is below the liquid/air level for the housing. The one or more ports that are below the liquid/air interface level may be used as sampling ports or access ports for probes or drains or supply ports for liquids or gases. The bioreactor provides a direct retrofit for the existing glass or steel assembly that utilizes the existing support structures and controls. The molded design overcomes issues of discontinuity, dead spots and the like due to its fixed dimensions that are built in by the molding process. Reproducible probe and other equipment location is also guaranteed through the use of the molded port features. The molded plastic allows for greater flexibility in material selection to reduce or eliminate lipid or cholesterol binding.