Prodromus of a fern flora of Bolivia. XLII. Update I. Author Kessler, Michael Systematic and Evolutionary Botany, University of Zurich, Zollikerstrasse 107, CH- 8008 Zurich, Switzerland. Author Smith, Alan R. University Herbarium, 1001 Valley Life Sciences Bldg. # 2465, University of California, Berkeley, CA 94720 - 2465. Author Øllgaard, Benjamin Department of Biology - Ecoinformatics and Biodiversity, Ny Munkegade 116, building 1137, 8000 Aarhus C, Denmark. Author Matos, Fernando B. Centro de Referência em Informação Ambiental (CRIA), Rua João Carlos do Amaral 500, 13070 - 111, Campinas, SP, Brasil. Author Moran, Robbin C. New York Botanical Garden, 2900 Southern Blvd., Bronx, New York, 10458 - 5126, USA. text Phytotaxa 2023 2023-12-20 630 3 183 210 https://phytotaxa.mapress.com/pt/article/download/phytotaxa.630.3.2/51376 journal article 10.11646/phytotaxa.630.3.2 1179-3163 10409920 Isoëtes Kessler & Smith (2018a) listed six species of Isoëtes for Bolivia but mentioned that probable undescribed species occur in the country. Since then, two publications ( Pereira et al . 2019 , Brunton & Pereira 2020 ) have rectified this situation, describing two new species from the country and clarifying their phylogenetic relationships. We here provide an updated key to the genus Isoëtes in Bolivia and species treatments for the two newly recognized species. The discovery of two new species of Isoëtes from shallow, seasonal pools on granite outcrops in eastern Bolivia suggests that further such microendemic species may occur on these island-like habitats, and this suggests that for further searches on such outcrops might be productive ( Brunton & Pereira 2020 ). The two new species were collected in November and March, suggesting that searches should be carried out in the rainy season and that the species may not be found in the dry season, when the pools dry. Further species of Isoëtes known from adjacent Brazil may also occur in eastern Bolivia ; these are described and keyed in Pereira et al . (2019) . A recent study on two Brazilian species of Isoëtes found that certain traits that have been used to characterize species in the genus are seasonally variable depending on water levels ( Cavalheiro-Filho et al . 2021 ). Variable traits include number of leaves, leaf length, sporangium size, number of megaspores, and megaspore diameter, whereas other traits such as color, ornamentation, and polar/equatorial diameter ratio of the megaspores, alae length, velum length and coverage, and morphometry of sporangial wall cells provide consistent characters for species delimitation. Such variation has not yet been studied among Bolivian species but should be considered when evaluating the taxonomic status of future collections.