SUMMARY
Two types of spherical or polyhedral virus-like particles approximately 33 and 44 nm in diam were found in morphologically unstable isolates of Alternaria tenuis. In cell-free preparations made from freshly cultured mycelium, only the 33-nm particles were represented. The role of these particles in determining morphological instability and its apparent transmissibility from “diseased” to “healthy” isolates is still unknown.