In the late summer of 1942, a Septoria sp. with spores longer than those of Septoria nodorum Berk., and conforming rather closely to those of S. Avenae Frank, was found commonly in the Prairie Provinces of Canada on leaves of wheat and, in one instance, on leaves of barley. It was found again on wheat and more rarely on barley in both Eastern and Western Canada each year from 1943 to 1946. Occasionally, perithecia, attributable to the genus Leptosphaeria, were found on leaves that previously had borne numerous pycnidia of this organism. That the perithecia represented the perfect stage of this Septoria sp. was shown by the development of similar perithecia in many cultures established from pycnidiospores, and by the formation of pycnidia in certain cultures grown from ascospores of perithecia collected on wheat. The perithecia, asci, and ascospores conform closely though not identically with the description of Leptosphaeria avenaria Weber.The organism under discussion differs from S. nodorum not only in the size of its pycnidiospores but also in its inability to cause glume blotch, its longer incubation period, and lack of ability to attack the seedlings of certain wheat varieties susceptible to S. nodorum. It differs from S. Avenae in symptoms, host range, length of incubation period, and cultural characteristics. It differs from both these species by a lower parasitic vigour and a marked tendency to develop on fading leaves and sheaths. Owing to its morphological similarity to L. avenaria it is here described as a forma specialis of that species.