The present invention concerns a plunging jet bioreactor (1) comprising a mechanism (4), (5) adapted in use to form culture fluid into a hollow flow stream and to introduce into the hollow of the flow stream an oxygen-containing gas. In some embodiments the mechanism comprises concentrically arranged outer (4) and inner tubes (5) wherein the inner tube is in flow communication with the culture fluid container whereby the culture fluid flows over the inner tube to form the hollow flow stream into which the oxygen from the inner tube is introduced.