The natural control exerted by Trissolcus basalis (Wollaston), a solitary egg parasitoid of Nezara viridula (L.), in soybean fields in central Italy over 6 yr has been assessed. T. basalis was the only egg parasitoid attacking N. viridula egg masses. In total, 260 N. viridula egg masses were collected. Of these, 105 contained at least 1 egg parasitized by T. basalis. Approximately 20% of egg masses were parasitized during the first 2 yr (1986 and 1987), increasing to 50% in the following years (1988, 1990, 1991, and 1992). Parasitism efficiency (the percentage of eggs parasitized over the total egg masses discovered) was =65% in 1986 and 1987, significantly less than in 1988 and 1990 (=92%). The impact of the parasitoid on the N. viridula population was low, <13%, for the first 2 yr, and much higher, =40%, in the following years.