A 4‐chamber proportional counter was built with very thin windows to detect weak beta radiation in an external gas cell. The instrumental design permits relatively high counting efficiencies and also incorporates an anticoincidence background reduction feature inherent in the detectors' position, with respect to spurious cosmic‐ray produced events. Gaseous materials containing the weak beta‐emitting radioisotopes Kr85, C14, and H3have been detected using this instrument. Over‐all counting efficiencies for these isotopes were 55%, 26%, and 1%, respectively. The specific activity of the material is calculated from PVT data in the sample cell. After calibration, the counter is useful as a means of secondary standardization or where impure gas samples must be tolerated.