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Many achnanthoid diatoms were transferred from Achnanthesor Achnanthidiumto new genera based on characteristics observed via scanning electron microscopy (SEM). In this study, we isolated and cultured two Korean freshwater achnanthoid diatoms (Bacillariophyceae), studied their morphological characteristics using SEM, and assessed their molecular characteristics by analysing the nuclear small subunit (SSU) and ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) genes. We emended the description of Lemnicola hungaricawith regard to the spindly pore opening of the hymen and the internally convex occlusions of elongated pores on the mantle. In addition, we demonstrated that the valves of L. hungaricaare occasionally biseriate to uniseriate towards the valve edge. A morphological comparison of Lemnicolastrains and species showed that biseriate rows of areolae are no longer an essential characteristic for allocation to Lemnicola. Based on the SEM and molecular analysis results, we also established a new, small-celled monoraphid species, Lemnicola uniseriatasp. nov., which has elongated areolae occluded by radiately structured hymenes and an elliptic valve shape. The uniseriate areolae of L. uniseriatashowed that Lemnicolaincludes both uniseriate and biseriate striae. Additionally, the results of SSU and rbcL sequence analyses provided strong evidence to support the recognition of L. uniseriataas a new species.