The forest tent caterpillar, Malacosoma disstria Hbn., began defoliating trembling aspen stands, Populus tremuloides Michx., in 1976 in the Turtle Mountains of North Dakota. Heavy defoliation was scattered through about 150,000 acres in 1978. Pupal mortality from parasites and disease was almost 100 percent in cocoons on understory shrubs and trees during July. However, biological evaluation in October showed egg masses present in tree crowns in 42 of 63 plots sampled throughout the infested area. Egg viability was high and averaged 91 percent. An average of 3 percent of the eggs were parasitized. Moderate defoliation is predicted for 1979 on 9 plots and light defoliation on the other 33 plots. We predict defoliation in 1979 will be spotty and light throughout the aspen stands of the Turtle Mountains, but several spots of moderate defoliation are expected around Lake Metigoshe and east of Carpenter Lake.