Measurements of opacity of particulate emissions by trained observer and by instack transmissometer were compared on 26 different occasions at 17 different stacks and 13 different plants to see how well these two commonly used methods would agree when applied to real plants under field conditions. During the study opacities ranging from 2 percent to 57 percent were measured. In general, agreement of + or - 10 percent opacity may be expected between observer and transmissometer measurements when observer measurements are at the stack exit and ambient conditions permit good visibility of plume.