Abstract The accidental introduction of the brown tree snake Boiga irregularis to Guam has resulted in the extirpation of most of the island's native terrestrial vertebrates, has created a health hazard to infants and children, and has resulted in economic losses. The high brown tree snake population densities on Guam, the species’ adaptations for successful dispersal, and Guam's position as a focal point for commercial and military cargo shipments have created a high level of concern that brown tree snakes could spread from Guam to colonize other vulnerable locations. Cargo inspections using teams of handlers and their detector dogs form the last line of defense for preventing brown tree snake dispersal from Guam. We assessed the efficacy of the teams of handlers and their dogs during 1998 and 1999 for locating stowed brown tree snakes by planting live brown tree snakes (in escape-proof containers) in cargo without the knowledge of the handlers inspecting the cargo. Many of these inspections were monitored by a concealed observer to determine the reason if a planted snake was not located by the dog teams. For 1998 and 1999, we found similar efficacies of 61% and 64%, respectively. When the snakes were not located by the dogs, we found that twice as many were missed because the dog did not change its behavior in response to the snake rather than because the handler did not conduct an adequate search pattern. During daytime inspections we found a greater efficacy for outdoor inspections than for indoor inspections. We found that the average efficacy of 62% for the 1998–99 period was lower than the baseline efficacy of 70% established in 1997.