A comparison was made between the drop size data obtained with a PMS FSSP-100 and a Calspan droplet sampler. Data were collected in simulated fogs in the Calspan environmental chamber and in natural fogs at the AFGL Weather Test Facility at Otis AFB, Massachusetts. Above 4 micrometers radius, the data from the two instruments agree quite well. Below 4 micrometers, however, the Calspan sampler shows a decrease in droplet count with decreasing radius whereas the FSSP shows an increase in count with decreasing radius. As a result, in natural fogs the Calspan sampler frequently shows a mode in the droplet concentration between 4 and 8 micrometers whereas the FSSP frequently shows no mode in droplet concentration existing within the drop size range of the instrument. In the simulated fogs, where the concentration of small particles was low, the modes agree very well. Since the droplets below 4 micrometers contribute little to the liquid water, there is excellent agreement in the distribution of liquid water with particle size for both simulated and natural fog. The extinction coefficients calculated from the FSSP drop size distribution are lower than the measured extinction coefficients in the lighter fogs but larger in the denser fogs.