Reviewed by William "Willy" Vizcarrondo, NYS DEC Certified Commercial Pesticide Applicator C1825832. Updated September 23, 2026.
Short answer
Water. Small dark brown ants in a bathroom are most often pavement ants, and Utah State University's extension notes a common space for their colonies is in bathrooms under toilets and showers, where there's moisture in the slab or the subfloor. They're attracted indoors by food, garbage and moisture, and they nest in foundation cracks and wall voids. Baits clear them; sprays push them somewhere else.
Because that's where the water is. Pavement ants nest in soil under slabs, in foundation cracks, along pavement edges and in wall voids, and Utah State notes that inside the house a common place for the colony is under toilets and showers. A toilet flange that weeps, a shower pan with a slow leak, a tub overflow that's never been sealed or a supply line that sweats keeps the slab or the subfloor damp, and the colony moves in beneath it and sends workers up through the gap around the drain, the flange or the baseboard. Utah State notes they're attracted indoors by food, garbage and moisture, or swarm indoors when they nest in or near foundation cracks or voids, and in a bathroom the moisture is the whole story. The same colony may have a second trail into the kitchen.
Pavement ants, most of the time. Utah State describes them as small, dark brown, about 2.5 to 3 millimeters long, with fine parallel grooves on the head and thorax, and outdoors they build small volcano-shaped mounds of dirt between pavers and along the edge of a walk or driveway. If the ants are the same size but give off a rotten coconut smell when you crush one, that's the odorous house ant, which also goes for moisture and sweets and gets the same treatment. If they're much bigger, a quarter inch or more and black, in a bathroom, that's carpenter ants in wet wood, and that's a different page and a different job.
Because the colony is under the floor, not on the tile. Utah State's guidance is that repellent products can cause ants inside the home to become disconnected from their colony, creating additional ant issues indoors; the sprayed trail scatters, the cut-off workers set up a new trail, and next week there are ants in two rooms. Baits go the other way. Utah State notes foraging workers carry bait back to feed the immatures and adults in the colony, and that a sweet liquid borax bait can eliminate activity in an area within about 5 days. The bait goes on the trail, near the drain or the baseboard where they emerge, and it's left alone. Then the water problem gets fixed, because a dry slab is a slab ants leave.
The bathroom ant call is a spring and summer regular in the slab houses of Levittown, Hicksville, Massapequa and Seaford, where the toilet flange or shower drain goes straight into a slab with a colony under it, and in the older Huntington and Babylon houses with a bathroom over a damp crawlspace. Bobby Bugs baits the trails, treats the entry points at the slab and the foundation, and finds the leak that's feeding them. Ant work carries a 90-day written guarantee.
Related
Under it. The colony is in the soil under the slab or in the wall void behind the fixtures, and the workers come up through the gap around the toilet flange, the shower drain, the tub overflow and the baseboard. Utah State names under toilets and showers as a common colony site.
Because cleaning doesn't touch the colony. Wiping the trail removes the scent for a day; the nest under the floor sends new workers. A bait that goes back to the nest, and a fix for the moisture, is what stops it.
Often, yes. One pavement ant colony under the slab can send trails to both, one for water and one for food. If you bait one trail and the other keeps going, the colony is still alive; bait both and the whole thing goes quiet.
Still not sure? Call (516) 509-6001 and we'll tell you what you're dealing with.