Longitarsus nigerrimus is generally rare across its range. It can be found across northern and central parts of Europe and Asia.
Longitarsus nigerrimus is restricted to wet boggy areas where its host plant (Utricularia) grows.
It requires areas with a seasonally fluctuating water level - high enough in spring and early summer for its host plant to grow, but falling in midsummer so its host plant is exposed above the water’s surface, usually supported on Sphagnum moss. It can only then begin to feed.