After Mike (Michael Garfield) and Don (Philip MacHale) get most of the killer slugs to a certain point in the sewer (with Don getting killed by them in the process), Mike gets out onto the street, and he and John (Santiago Alvarez) put a large quantity of a lithium-based arsenic down into the sewer. Since lithium is highly combustible when it comes into contact with moisture, the explosion that results after it hits the water kills all of the slugs in the entire sewer system.

However, the camera eventually shows one still-living slug crawling on a sewer grate….

Thanks Tornado Dragon!