Dilute solutions of several basic dyes, especially Crystal Violet and Malachite Green, effectively marked cocoons of Neodiprion sertifer (Geoff.). In field experiments, 0.25% Crystal Violet applied to soil at 1.0 gallon per square foot of surface area dyed most N. sertifer cocoons recognizably and was not removed by natural leaching or by the water used in extracting cocoons from the soil. Cocoons spun later in treated soil became contaminated but could be recognized by the distribution of dye on detritus adhering to the surface. Dyeing permitted direct measurement of the density of current generation cocoons in late August, when they were visually indistinguishable from old ones.