The effectiveness of poisonous substances often decreases after they have been in use for some time. This phenomenon, which was apparently known to Theophrastes (Inquiry into Plants, Book IX, Chapter XVI), has caused considerable difficulty to modern man in his efforts to control harmful organisms by the use of toxic compounds. Man has developed very effective insecticides and has been able to apply them economically against insects harmful to agriculture or to public health. However, insects have often developed insecticide-resistant forms which continue the struggle. At least 137 species of insects, mites, etc., developed tolerance to various groups of insecticides by 1960 (Brown, 1961). Antibiotics have been commercially produced and extensively used in the chemotherapy of many bacterial diseases of man and animals. In this case, also, development of resistance disappointed the optimists who believed that, with the use of antibiotics, infectious diseases would become a thing of the past. Familiar with such experiences, the average reader of this article may be surprised to hear that in the control of fungal diseases of plants fungicide resistance has not, to the present time, created comparable problems. The appearance of resistant strains in the field has occurred in only a few cases. The resistance problem seems to have been minimized mainly because of the nonspecific nature of the fungicides used to control plant diseases. From the selectivity point of view, we are far behind in the field of fungicides as compared to bactericides or insecticides. This, of course, will not remain so and, in fact, there is now good reason to suggest that selective fungitoxicants will soon be introduced and become very important in agricultural practice. Fungicide resistance may then become of much greater practical concern and, therefore, knowledge in this field should prove valuable. Fungicide Resistance in the Field