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ABSTRACTA new species of NitzschiaHassall is described from a small spring in southern Portugal. Nitzschia transtagensissp. nov. has a unique morphology in the Linearessection, resembling Nitzschia hierosolymitana, N. vitreaand its variety salinarum. The new species is unique in that it has a convex primary (raphe) side with a slightly undulated abvalvar edge of the mantle, and a clearly undulated secondary valve side. This taxon also has a dimorphic open, ligulate cingulum composed of an apically asymmetric valvocopula having one row of roundish pores on the pars interior and attached to the valve secondary side, and two rows of pores (one of roundish and one of elongate pores) attached to the primary side of the valve. All remaining girdle elements have a single row of approximately round pores. The new species is compared to morphologically similar taxa and its ecology is discussed based on available information and literature. Additionally, it was found that a variant of the new species had been reported previously from Sardinia as “Nitzschiaaff. ebroicensis”, but the Italian population has coarser stria and puncta densities.