Abstract
Isoetes tamaulipana, an aquatic plant that grows in small pools formed in outcrops of granitic rocks, and currently known only from the municipality of San Carlos in the state of Tamaulipas México, is described and illustrated. The new species is compared with taxa that are growing in similar environments conditions or have recently been described from the New World. Isoetes tamaulipana differs from these other taxa (e.g. I. lithophila, I. orcuttii, I. texana, etc.) in having megaspores with smooth perispore and microspores with echinate perispore.