An instrument is described which will detect and record the alpha‐disintegration rate of prepared radioactive samples even though soft beta‐radiation up to 5×109 disintegrations per minute may also be present. The counting losses arising from the detector, electronic amplifier, scaler, and recorder total 0.8 percent with an alpha‐counting rate of 105 counts per minute. The instrument employs a proportional counter through which 99 percent CH4 at one atmosphere slowly flows. The proportional counting region is approximately 300 volts in length, with 0.5 percent change in counting rate in an interval of 200 volts. Thin radioactive samples deposited on metal disks up to 2.5 cm in diameter are easily inserted into the counter volume by means of a special gas lock. The counter detects 0.50 of the disintegrations from a thin radioactive sample. The counter background can be maintained at about 14 counts per hour. A high vacuum, low geometry attachment provides reductions in geometry down to at least 10−5 of the sample counting rate. Some theory of gas‐flow proportional counters is presented along with details of the instrument construction.