Abstract: A collapsible bag sampler comprises a top section; a bottom section connected to the top section; a tail section having fins, which tail section is fitted onto the top section; a nose section with tray, which tray supports a flexible bag; a nozzle holder; a nozzle holder insert in the top section to hold the nozzle; and a flexible bag attached to the outside of the rear of the nozzle holder. Because the bag container is flexible and contains substantially no air, there are no limitations due to air compressibility, meaning the depth to which the sampler could be used would be limited only by the size of the intake nozzle and the volume of the bag. The transit rate is limited only by the apparent approach angle of the nozzle facing into the stream velocity as it makes its vertical descent and ascent (0.4 times the stream velocity). The sampler has a useful range of from 2.0 ft/sec to approximately 10 ft/sec stream velocities.