Abstract: A fiber optic bulkhead penetrator for allowing fiber optic communication lines to penetrate through an undersea bulkhead employed in offshore oil well exploration and production apparatus having a very high pressure differential thereacross. The arrangement consists of a port and cofferdam installed in the undersea oil well exploration and production apparatus, and a bullnose penetrator, having a plurality of bulkheads therein, terminating an undersea fiber optic cable used for data transmission. The bullnose penetrator serves as a mating plug for a port provided in the oil well apparatus, with three sets of seals isolating the single-atmosphere environment from the ambient seawater pressure. A shoulder is provided on the penetrator and in the port to hold the penetrator in place once the cofferdam is evacuated and lowered to habitat pressure.