A widely distributed species occurring across North Africa and the Caucasus, throughout Europe north to the Arctic Circle and east into Russia; through much of central Europe it is the most common member of the genus, occurring up to 2000m in mountain regions. In the UK it is locally common and often abundant across England and Wales, becoming more sporadic further north to the Scottish Highlands. Adults occur under bark of a wide range of broadleaf and coniferous trees in various stages of decay, especially where fungus is present, usually in numbers and often in company with R. bipustulatus. Larvae have been reared on a wide range of fungi e.g. Fomes frumentarius (L.) Fr., Trichaptum abietinum (Dicks.) Ryvarden (1972) or various Inonotus (Bull.) P. Karst (1879) but they are also predatory and have been reared on dead larvae of bark beetles. Adults will consume mycelia but are predominantly predatory on scolytid eggs and larvae; on the continent associated especially with Tomicus piniperda (Linnaeus, 1758) and T. minor (Hartig, 1834) but also Pityogenes chalcographus (Linnaeus, 1760), Dendroctonus micans (Kugelann, 1794) and various species of Ips De Geer, 1775 and they are attracted to a range of bark beetle pheromones as well as ethanol. The species is among the most important control agents of bark beetles but also a vector of various fungal plant pathogens. Adults occur very early in the year and are present into the autumn, they occur under damp or wet bark, usually around damaged areas on where fungi are present, and may be observed on the surface at night, early in the year we have extracted them from moss and tussocks growing on fallen timber and logs and at this time they also occur in flight-interception traps.

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2.5-4.5mm. Mature adults have a distinctive elytral colouration; black with the base and apex extensively brown, this is also seen in R. nitidulus (Fabricius, 1798) but here the middle tibiae are strongly sinuate towards the apex. Body and appendages brown with the pronotal disc darkened at least to some extent and often extensively so. Head relatively large, across the eyes as broad as the broadest part of the pronotum, with strongly convex and protruding eyes, long and narrowed temples and 2 oblique depressions on the frons. Antennae 11-segmented and inserted laterally in front of the eyes, the third segment as long as the fourth and fifth combined.  Head and pronotum with elongate and moderately large punctures. Pronotum elongate, broadest anteriorly and narrowed to the base, anterior angles rounded, not produced forward, lateral and basal margins strongly bordered. Elytra with prominent shoulders, broadest about the middle and evenly curved to separately rounded apices, compared with R. bipustulatus more strongly tapering towards the apex and lacking the oblique sub-basal impressions. Strial punctures strong but becoming weaker towards the apex, interstices without punctures. Legs long and robust, the outer margin of all tibiae straight; middle tibiae with several small spines towards the apex.