THE ASCOSPORES OF some species of Ascobolus, a genus of operculate discomycetes, have proved difficult to germinate in -the laboratory (Dodge, 1920; Dodge and Seaver, 1946). This difficulty has impeded a thorough study of the life histories of these interesting fungi:. Consequently, the chief purposes of this investigation were to develop a method that would induce good spore germination, and to study the factors influencing sporulation on artificial media. Five species of Ascobolus, namely A. magnificus Dodge, A. stercorarius (Bull.) Schr6t., A. immersus Pers., A. winteri Rehm. and A. viridulus Phill. & Plow., were isolated from various kinds of animal dung. A. magnificus was first described by Dodge in 1912. In 1920 he reported it as heterothallic, the first demonstration of clear-cut heterothallism in the ascomycetes. There is no visible difference between the spores of the two mating types. However, the writer has observed that there is a difference in rate of growth of the two compatible strains. One strain (which we call A) grows more rapidly than the other (which we call a),. This same difference in growth rate was observed in A and a mating types of two different sets of isolates from nature and in the progeny resulting from crosses between these isolates. In culture, numerous papulospores are produced. These were shown by Dodge (1920) to be asexual spores of A. magnificus. Rizet (1939) proved that A. immersus also is heterothallic. Green (1931). and Ames (1930) found that some single-spore cultures of A. stercorarius and A. immersus gave rise to apothecia, but they did not indicate whether these apothecia contained ascospores. Dowding (1931) believed that such apothecia resulted from oidial contamination caus'ed by mites and flies. We have never obtained fertile apothecia in single-ascospore cultures of any of the heterothallic species. A. winteri and A. viridulus are homothallic. All five species of, Ascobolus studied by the author have been cultured in the laboratory on their natural substrates by various workers (Dodge, 1912a, b; Dangeard, 1907; Dowding, 1931; Ramlow, 1915; Rizet, 1939). Green (1931) cultured A. stercorarius on M medium (0.1 per cent MgSO4, 0.1 per cent K3PO4, 0.1 per cent N114NO3, 0.01 per cent Na2CO3, 3 per cent agar, 0.25 per