In a previous study, ants had a significant effect on gypsy moth,Lymantria dispar(L.), numbers in a forest where the trees were only about 3 m high. The present study was carried out similarly, but trees were about 20 m high. In some plots, ant numbers were suppressed by a combination of barriers and poison baits. In other plots, 24% sucrose sprays were applied with a back-pack mist blower in an attempt to increase ant abundance, but no increases occurred. There was a nonsignificant trend for caterpillar numbers to be higher in the plots where ants were suppressed, and gypsy moth larval counts on leaves of trees were negatively correlated with ant counts on leaves. Thus, there is evidence that ants have an effect on gypsy moth caterpillars in mature forests as well as in young ones.