Invasion of the adult roundworm, Ascaris lumbricoides, into the gallbladder is rare and was seen in 14(2.1%) of the 665 cases with hepatobiliary ascariasis. The diagnosis was suggested in all 13 cases in which sonography was performed and in 5 of the 11 cases at endoscopic retrograde cholangiography. Sonographic findings included a nonshadowing, long, echogenic structure in the form of a coil, an echogenic strip with central anechoic tube, an echogenic structure extending across the gallbladder giving it a septate appearance, and characteristic erratic, nondirectional, zig-zag movements of these echogenic structures in the gallbladder. Serial sonograms accurately predicted spontaneous exit of the worm. Pregnancy and anomalous origin of the cystic duct directly from the papilla of Vater facilitated worm invasion into the gallbladder. We conclude that real-time sonography offers a simple, rapid approach for the diagnosis and follow-up of patients, whereas endoscopic retrograde cholangiography has limited diagnostic value in this disorder.