Boron trimethyl counters have operated successfully as neutron sensitive proportional counters. Differential pulse‐height spectra for these counters show good resolution. The electron mobility in boron trimethyl is measured by the method of Nicholson in theX/Pregion from 0.8 to 7.0 v/cm/mm Hg. The data indicate a constant mobility aboveX/P=2.5 volts/cm/mm Hg but the mobility increases rapidly for lower values.