A continuous wave coherent laser radar with an output power of 2 W at 1.5 µm was used to measure the backscattering from a bio-aerosol. The aerosol particles used in this experiment were generated from Turex, a non-pure technical preparation of spores of Bacillus thuringensis (BT). A series of measurements in a laboratory environment were performed. The distance to the aerosol was 28 m. The aerosol was maintained in a shower cabin incorporating a tube arrangement to circulate an air stream with the particles. A particle counter was connected to the cabin and particles in the size range from 0.5 to 10 µm were counted. A detectable backscattering signal was found for a total particle concentration ranging down to 4000 particles per liter (ppl). The backscatter signal was quantified in different ways. An average value of the backscattering efficiency for the particles, assuming it to be independent of size, was estimated to be Qp ~ 1 with an uncertainty of a factor of four. The corresponding detection threshold for particles with a typical size of 2 µm is about 2000 ppl. It was realized that the detection threshold will be a function of the particle radial velocity distribution which is a potential drawback of the coherent laser radar for this application in practice. On the contrary, the detection threshold can in principle be shown to be independent of range and aperture size as long as the particles are uniformly distributed within the measuring volume. However gauging at longer ranges might require a larger optical aperture than the 30 mm diameter used in these tests in order to confine the focal length to the target cloud.