A method of milling chlorothalonil to form a sub-micron product having a narrow particle size distribution is presented. The milling involves wet milling of the chlorothalonil with high density milling media having a diameter between 0.1 mm and 0.8 mm, preferably between 0.2 mm and 0.7 mm, and a density equal to or greater than 3.8 g/cc, preferably greater than 5.5 g/cc, in a ball mill using between about 40% and 80% loading of the mill volume with milling media, and having the chlorothalonil suspended in an aqueous milling liquid which comprises one or more surface active agents. The milling speed is preferably high, for example from about 1000 rpm to about 4000 rpm. The milled product can be used in foliar applications at a lower effective dosage than prior art formulations, can be used in improved antifouling paint formulations, and can be used in new applications such as the direct injection of solid chlorothalonil particulates in wood to act as a long lasting wood preservative.