Ilybius fenestratus is a species of beetle found throughout Europe and Northern Asia.

[1] The scent gland of this species of beetle is natural source for the anabolic steroid boldenone (Δ1-testosterone).

[2] A study conducted in Målsjøen found the life cycle of the species to be semivoltine, with adults emerging in midsummer.

Breeding takes place from mid- to late summer, the last adults dying in autumn.

The larvae hatch in late summer and hibernate, in the water, until April to May, when they emerge from the water to pupate.