A new bacterial disease of a cultivated mushroom, Agaricus bitorquis, is reported. Symptoms exhibited by affected sporocarps range from mild blotching to deep pitting, where large pervasive cavities extending from the cap surface to the stipe may form. Sporocarps often show a characteristic eroded appearance where the tissues are completely degraded. Some areas of diseased sporocarps fail to show any external symptoms, but on cutting the cap, massive breakdown of the underlying tissue is evident, resulting in a hollow cap. The causal bacterium, confirmed by application of Koch's postulates, is a nonfluorescent pseudomonad identified as Pseudomonas cepacia. Key words: Pseudomonas cepacia, Agaricus bitorquis, tissue degradation, cavities.