A trap for a catch basin, and means for mounting said trap to a catch basin wall, comprises a hollow trap body having an inner wall and an outer wall, and side walls joining the inner and outer walls. The trap body has a first aperture near the top of its inner wall, allignable with an outlet of a catch basin, and a second aperture at a lower level on said trap body than said first aperture. A mounting plate is secured to the inner wall of the trap body, and has an aperture allignable with the first aperture of the trap body. A retainer plate is provided, onto which the mounting plate can be mounted. The retainer plate also has an aperture allignable with the apertures on the mounting plate and trap body.