mephiticus is a diminutive summer annual growing to a height of 15 cm (usually much smaller, especially at high elevation).

The oppositely arranged leaves are linear or lance-shaped and up to 3 centimeters in length.

The leaves emit an unpleasant scent when crushed, the characteristic that earned the plant its name.

The flower is magenta or yellow, and populations occasionally have plants with both colors.

The lower lip of the flower is marked with purplish lines and spots.