Fifty-two Gram-negative strains were isolated from Passalid (approximately 85 % of the samples) and Cerambycidae beetles in Amazonian forests near Itacoatiara, Brazil. Samples included hemolymph, larval surface washes, adult abdominal washes, gallery swabs, and live adult intestinal contents. Screening against Bacillus cereus ATCC 11778 and Escherichia coli ATCC 25922 identified three most active isolates—4 (H 11.1.3), 43 (LV 4.1.1), and 50 (H 11.1.1). Isolates 4 and 50 originated from Cerambycidae larval hemolymph; isolate 43 from a Cerambycidae larval surface wash. All three produced inhibition zones > 15 mm against both testers. Crude 48 h LB fermentations of these isolates also inhibited E. coli, B. cereus, and Staphylococcus aureus (halos 14–18 mm). Colony morphology: isolate 4 formed translucent light-caramel, creamy, mucoid colonies; isolate 43 formed translucent caramel-orange colonies; isolate 50 formed translucent orange-caramel colonies. Although further work was curtailed by funding constraints, these findings suggest Cerambycidae larval niches (hemolymph and surface) harbor antibiotic-producing Gram-negative symbionts.