Microcoleus chthonoplastes is a filamentous, nonheterocystous cyanobacterium which fixes gaseous nitrogen in axenic culture under aerobic conditions. Growth on agar plates reflects the benthic habit observed in the natural environment, and in liquid culture clumps or aggregates are produced. Scanning electron microscopy suggests that these aggregates are primarily the result of random movement of trichomes against one another. The aim of this study was to ascertain whether aggregation or any intracellular differentiation might be important in protection of the oxygen-labile nitrogenase in this species. Observations using light and transmission electron microscopy revealed no cellular or intracellular differentiation, and tetrazolium labelling experiments provided no evidence that a proportion of cells might act in a heterocyst-like fashion. The results presented suggest that nitrogen fixation in M. chthonoplastes is unlikely to be enhanced in oxygen-deficient microsites formed as a result of aggregation.