Equisetum X ferrissii Clute (Equisetaceae) is the sterile hybrid between E. hyemale L. var. affine (Engelm.) A. A. Eaton and E. laevigatum A. Br. A herbarium and field study of E. X ferrissii 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. Equisetum X femissii is much more widespread and abundant than previously realized.