Sphaerodes quadrangularis Garcia, Stchigel & Guarro was shown to be a facultative contact biotrophic mycoparasite of Fusarium avenaceum (Fr.) Sacc. (teleomorph: Gibberella avenacea Cooke) that established an intimate relationship with its host by producing hook-shaped and clamp-like attachment structures that appeared to derive nutrients and other essential factors from living host cells. The contact cells of the mycoparasite surrounded and envelop host hyphae and, although they do not appear to penetrate the host, the parasitized hyphae were significantly smaller than unparasitized controls. Sphaerodes quadrangularis did not form specialized attachment structures when cultured together with strains representing F. oxysporum and F. graminearum, indicating some degree of host specificity in the parasite.