Tape recordings made in the helium‐oxygen atmosphere of the U. S. Navy'sSEALAB IIhave been analyzed by spectrographic techniques. The divers breathing this gas mixture have an unusual voice quality often characterized as “Donald Duck” speech. The Sealab experiment permitted speech analysis of divers living in such an atmosphere for several days. A preliminary study of the recorded data has led to the following observations: (1) formant shifts are responsible for the unusual quality of the helium‐oxygen speech; (2) the formant shifts are nonlinear—the frequency shift of the first formant is greater than the frequency shifts of the higher order formants; (3) relative formant amplitudes are often changed radically; (4) pitch or fundamental frequency changes are not significant; (5) after several days in this new atmosphere, changes occurred in the speech quality that made it sound more natural. These changes most likely have been affected by modifications of the vocal‐tract gestures.