Abstract: A sonobuoy having its component parts packaged within a cylindrical housing uitable for dispensing from an aircraft launcher and automatically deployable to a transponder mode in its descent through the air and water. On launching, a wind flap at one end of the housing opens to pull a lanyard attached to a drag chute retaining bag which is pulled off to release a chute. Upon water immersion, a circuit sensitive thereto energizes the sonobuoy electronics and fires a squib which releases compressed CO.sub.2 to extend an inflatable float bag to the outside of the housing. A bag retainer cap applies an outward force to a payload release mechanism jettisoning the chute and permitting the float bag to inflate, erect a flexible antenna therein, rise to the surface, also disengages the housing permitting it and components which are pulled out of the housing therein to descend freely in the water paying out a suspension and signal cable to a desired hydrophone operating depth.