Several proportional fission neutron counters are described which use a uranium compound as the neutron sensitive material coated on the inside surface of the cathode. The counters are filled with high purity argon gas which produces very fast, clean pulses. The instruments use a discriminator circuit which eliminates the α-pulses from the uranium and also makes the instrument insensitive to γ-rays. A portable circuit has been developed consisting of two inverters, a discriminator, and a multivibrator. Subminiature tubes are used because of their small size, low current drain, and relatively high amplification factor. A laboratory model of the slow neutron instrument had an efficiency of 0.01 percent for polonium beryllium neutrons.