Abstract
Floodplains are heterogeneous ecosystems that occupy a large area in the neotropic regions, which show great diatom diversity. During a floristic survey of periphytic and sedimentary samples in a swamp from the Upper Paraná River Floodplain in Brazil, a new species of Actinella Lewis and Gomphonema Ehrenberg (Bacillariophyceae) were found. We detailed their morphology using light and scanning electron microscopy. Actinella hermes-moreirae sp. nov. is distinguished by its small valve lengths, low length to width ratio, moderately arched valves, absence of spines and the slightly arched termination of the raphe. Gomphonema mutunensis sp. nov. differs by the rostrate and sub-rostrate headpole, attenuated-rounded footpole and differently-shaped areolae.