The profundal macrobenthos of a regulated lake (Lake Canterno, Central Italy) was investigated to provide information on the structure and composition of this community in a lake long subject to wide fluctuations of the water level and to the influence of domestic sewage. Pollution‐tolerant forms constituted most of the total macrobenthos. It is suggested that large water movements (water level drawdown and hypolimnical drawoff), by producing a marked environmental instability, and eutrophication phenomena (e.g.deoxygenation), account for: a) high abundances of mobile fauna(Dero digitata, ProcladiusandChaoborus flavicans),especially at the station close to the water outlet; and b) high abundances of mud‐dwelling fauna (Tubificidae andChironomus plumosus)in the stations far from the water outlet. Diversity was negatively affected by mobile form percentages, and positively influenced by mud‐dwelling fauna percentages. Lastly, behavioural indicators are suggested to be useful markers with which to monitor environmental stress in lakes.