The most universal detector for molecular beams is an electron bombardment detector But, usually, in this detector molecular beam signal is obscured by the background and noise from the residual gas. By means of beam modulation and lock-in amplification the beam signal can be separated from the background and amplified with the improvement in signal to noise ratio. A detector of this type was constructed in our laboratory and its performance was studied with a molecular beam of naphthalene. The detector is a kind of an ionization gauge. Sensitivity of the detector as a ' vacuum gauge was 2×10-2 A/torr · mA for the residual gas. Beam signal of about 10-2 of the residual background gas signal was measured with S/N ratio of 2-3×102. Ionization efficiency of beam molecules was the order of 10-5 with electron current of 0.1 mA and a signal of 10-12A could be measured with the S/N ratio. of 1. This ion current corresponds to the beam intensity of 1012molecules/ sec.Beam profiles and the dependence of the beam intensity upon the oven temperature were measured. From the latter measurement heat of sublimation of naphthalene was estimated as 18. 6 kcal/mole and 19.8 kcal/mole for two different runs. The values are reasonable compared with the value 18. 8, 18. 9 kcal/mole calculated from the vapor pressure of naphthalene.