<em>Equisetum</em> X <em>ferrissii</em> Clute (Equisetaceae) is the sterile hybrid between <em>E. hyemale</em> L. var. <em>affine</em> (Engelm.) A. A. Eaton and <em>E. laevigatum</em> A. Br. A herbarium and field study of <em>E.</em> X <em>ferrissii</em> was undertaken to determine the range and abundance of this hybrid in Illinois. The hybrid occurs in a wide variety of habitats in the state but is most frequent in moist, open, disturbed habitats especially in sandy soils and along railroads. <em>Equisetum</em> X <em>femissii</em> is much more widespread and abundant than previously realized.