Abstract: A flask for establishing and maintaining environmental control for microbial cultures. The flask has a body resembling an erlenmeyer flask, and a screw capped large opening in the top thereof. The flask also has three sidearm tubes extending through its walls, one for outlet of gas from the flask, and another, extending into the flask to a point near its bottom, which functions as a gas input conduit. A third sidearm tube is provided for inoculation of the flask and sampling of its contents. Each sidearm is provided with a septum-type plug and a perforated screw cap to hold the plug in sealing position. The plugs and caps permit aseptic injection and withdrawal of gas or liquid to and from the flask by means of hypodermic needles inserted through the caps and plugs without disturbing the atmospheric conditions established within the flask. Methods for use of the apparatus for establishing and maintaining aerobic and anaerobic environmental systems are also disclosed.