Establishing the geographical distribution limits of species may provide not only data to understand their range of adaptation to conservation proposals but also records of the dynamics in the vegetation across the time. Here we report the southernmost limit of the occurrence of Isoetes panamensis Maxon & C.V. Morton within a relict of the Cerrado in Paraná, Brazil. This find highlights the importance of fieldwork in relicts of the Cerrado to enhance the knowledge about the flora of Paraná and the vegetational dynamic of the Cerrado in the past.