Abstract The general morphological features of the Gram-negative bacterium Pseudomonas avenae have been studied by electron microscopy. Electron micrographs from negatively stained specimens show the outermost surface layer to be composed of subunits arranged in a crystalline tetragonal array, with an average lattice spacing of 6.7 nm and unit cell of 9.7 nm when determined by image processing methods. Experiments designed to show the effects of various negative stains on the bacterial cell wall morphology suggest that the ‘convoluted’ or ‘smooth’ appearance of the wall material can be influenced by different stains when mixed with cells in liquid suspension. Various morphological products assembled as extracellular structures from subunits apparently of about the same size as those forming the cell surface crystalline lattice were observed in specimens prepared from a solid growth medium. These extracellular products ranged from tube-like assemblies with considerable variation in length, to forms which consisted of slender filaments in a coiled configuration. The axial periodicity of some of these products was 6.7 nm.