The tachinid Aplomyiopsis xylota parasitizes larvae of the goldenrod beetles Trirhabda virgata and T. borealis. The intensity of parasitism varied with larval stage and time of year. Parasitism of 3rd-instar larvae, the primary host stage, was high (70 to 90%) early in the season when T. borealis was vulnerable. Rates of parasitism dropped later in the season when most T. virgata larvae were sufficiently mature to elicit parasitoid attack. Percent parasitism was inversely correlated with host density, suggesting that the parasitoid's capacity was saturated when hosts were abundant. The susceptibility of larvae did not vary by which goldenrod species served as the larval food plant