Bacteria attached themselves rapidly and firmly to Agaricus bisporus. The percentage of artificially applied cells of the pathogenic bacteria Pseudomonas tolaasii and Ps. gingeri (n. sp.) which became attached to the fungus was twice that of non-pathogens. The percentage detectable attachment of Ps. tolaasii was 75% and that of Ps. gingeri 65%, whereas that of Ps. reactans was 31%, Ps. marginalis 19%, Ps. putida 31% and Escherichia coli, 23%. When examined by scanning electron microscopy, the arrangement of the cells of Ps. tolaasii on the A. bisporus hyphae was the same, before and after washing with distilled water. Hyphae growing on agar, and on the outer surfaces of mushroom caps, artificially inoculated with Ps. tolaasii, as well as naturally infected mushroom tissues, showed two features in common. Firstly, some areas of hyphal surface were seen to be free of bacteria. Attachment of bacteria is not a uniform occurrence all over the fungus. Secondly, the attached cells of Ps. tolaasii were apparently connected to the surfaces of the hyphae of the fungus, and to each other, by rod-like connections, variable in thickness, but rarely more than 0·1 μm in diameter. Drying the material in air for 24 h instead of fixing it in glutaraldehyde, before coating with gold-palladium, had no effect on the appearance of these connecting structures.