ABSTRACT. – Diversity and assemblage structure of water beetles sampled from twenty eight streams covering northern Tunisia have been analyzed. In addition to habitat and spatial variables, several physical and chemical variables were measured at each site. Samples from mountainous streams were dominated by rheophilous species of the family Hydaenidae and the subfamily Hydroporina while lowland streams are colonized by limnophilous species belonging to the families Hydrophilidae, Haliplidae and Dytiscidae. The most abundant taxa were Berosus affinis, Haliplus lineaticollis, Laccobius atratus, Hydroporus feryi, Laccophilus minutus, L. hyalinus and Hydraena leprieuri. Species richness revealed seasonal and spatial variation. Non-parametric multidimensional scaling (NMDS) ordination highlighted the faunistic affinities between sites revealing five main groups. Species-environmental variables relationships were examined using principal component analysis (PCA) showing two main species groups. Conductivity, water temperature, pH, dissolved oxygen, water permanence, current velocity, substrate type, altitude and abundance of aquatic vegetation were identified to be the major drivers structuring beetle assemblages and influencing species abundance and richness. These results suggest that habitat heterogeneity was the predictor of beetle assemblages, while species richness could be expected at a landscape scale using physicochemical features.